Charcoal, 37 bytes

≔⁺⭆χιβη¿∨¬θ⊙θ¬№ηιInvalid inputＦηＦ№θιι

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≔⁺⭆χιβη

Concatenate the digits with the lowercase letters.

¿∨¬θ⊙θ¬№ηι

If the input is empty or contains a character not in the digit letter string, ...

Invalid input

... then output the required error message, ...

Ｆη

... else for each digit and lowercase letter...

Ｆ№θιι

... output each time it appears in the input.

x86-16 machine code, IBM PC-DOS, 81 80 bytes

Binary:

00000000: b40a ba4f 01cd 218b f2ad 8bd6 8acc 8be9  ...O..!.........
00000010: 938b f28b fe51 ad3c 307c 0c3c 397e 0e3c  .....Q.<0|.<9~.<
00000020: 617c 043c 7a7e 06ba 4201 59eb 103a c47c  a|.<z~..B.Y..:.|
00000030: 0286 c4ab 4e4f e2de 59e2 d688 1bb4 09cd  ....NO..Y.......
00000040: 21c3 496e 7661 6c69 6420 696e 7075 7424  !.Invalid input$

Build and test DISSECT.COM using xxd -r

Unassembled listing:

B4 0A           MOV  AH, 0AH            ; DOS read buffered input from STDIN 
BA 014F R       MOV  DX, OFFSET BUF     ; load buffer pointer    
CD 21           INT  21H                ; get STDIN input 
8B F2           MOV  SI, DX             ; buffer pointer into SI 
AD              LODSW                   ; load char count into AH 
8B D6           MOV  DX, SI             ; save start of string 
8A CC           MOV  CL, AH             ; string length into CX 
8B E9           MOV  BP, CX             ; save string length for later display 
            OUTER_LOOP: 
8B F2           MOV  SI, DX             ; reset SI to beginning of buffer 
8B FE           MOV  DI, SI             ; reset DI to beginning of buffer 
51              PUSH CX                 ; save outer loop counter 
            INNER_LOOP: 
AD              LODSW                   ; load next two chars into AH/AL
3C 30           CMP  AL, '0'            ; is char < '0' ? 
7C 0C           JL   INVALID            ; if so, not valid 
3C 39           CMP  AL, '9'            ; is char <= '9'? 
7E 0E           JLE  VALID              ; if so, valid 
3C 61           CMP  AL, 'a'            ; is char < 'a'? 
7C 04           JL   INVALID            ; if so, not valid 
3C 7A           CMP  AL, 'z'            ; is char <= 'z'? 
7E 06           JLE  VALID              ; if so, valid 
            INVALID: 
BA 0142 R       MOV  DX, OFFSET INV     ; load address of "invalid" string 
59              POP  CX                 ; discard saved counter 
EB 11           JMP  OUTPUT             ; display "invalid" string
            VALID: 
3A C4           CMP  AL, AH             ; compare chars 
7C 02           JL   NO_SWAP            ; if AL < AH, do not swap 
86 C4           XCHG AL, AH             ; otherwise, swap chars 
            NO_SWAP:
AB              STOSW                   ; store chars back to buffer 
4E              DEC  SI                 ; adjust SI to previous char 
4F              DEC  DI                 ; adjust DI to previous char 
E2 DE           LOOP INNER_LOOP         ; loop inner iterations
59              POP  CX                 ; restore outer loop counter 
E2 D6           LOOP OUTER_LOOP         ; loop outer
C6 03 24        MOV  BYTE PTR[BP+DI],'$'; add string terminator
            OUTPUT:
B4 09           MOV  AH, 09H            ; DOS display string function 
CD 21           INT  21H                ; write output to console 
C3              RET                     ; exit to DOS

INV DB "Invalid input"                  ; "invalid" message string
BUF DB '$'                               ; buffer size is 36 bytes, also string terminator

Standalone PC DOS executable. Implements a simple bubble sort algorithm.

Input via console/STDIN, output to console/STDOUT.

Perl 5 -pF//, 45 bytes

$_=/^[a-zd]+$/?join"",sort@F:"Invalid Input"

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PHP, 92 bytes

$a=str_split($s=$argn);sort($a);echo preg_match('/^[a-zd]+$/',$s)?join($a):'Invalid input';

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Burlesque, 37 bytes

><JJann!/^zz!=||{vv"Invalid Input"}if

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><                    # Sort input string
JJ                    # Duplicate twice
ann!                  # Not alphanumeric
/^                    # Swap and duplicate (sorted string)
zz!=                  # Lowercased not equal to original (i.e. contains uppercase)
||                    # Or
{vv"Invalid Input"}if # If not alphanumeric or lowercase drop sorted list 
                        and output invalid input

Jelly, 18 bytes

ɠfƑØBŒl¤ȧṢȯ“4{ẉṄi»

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A full program that reads its input from STDIN and outputs to STDOUT.

If the input validation and requirement to use STDIN were removed, this would be a single byte: Ṣ

C++ (clang), ^{223 $cdots$ 182} 170 bytes

Saved 8 a whopping 20 bytes thanks to ceilingcat!!!

#import<bits/stdc++.h>
int main(){std::string s,i="Invalid input";getline(std::cin,s);sort(begin(s),end(s));for(int c:s)s=c<48|c>122|c>57&c<97?i:s;std::cout<<(s[0]?s:i);}

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A program that reads a single string from stdin (handling all invalid cases) and outputs to stdout.

Red, 86 bytes

func[s][a: charset[#"0"-#"9"#"a"-#"z"]either parse sort s[some a][s]["Invalid input"]]

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APL (Dyalog Extended), 37 bytes^SBCS

{0∊(≢⍵),⍵∊⎕D,⌊⎕A:'Invalid input'⋄∧⍵}⍞

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⍞ prompt for text input from the console

{…} apply the following anonymous lambda to that:

 ⎕A the uppercase Alphabet

 ⌊ Lowercase that

 ⎕D, prepend the digits

 ⍵∊ for each character of the argument, indicate if it is a member thereof

 (…), prepend the following:

  ≢⍵ the tally of characters in the argument

 0∊…: if zero is a member thereof:

  'Invalid input' return (and implicitly print) this message

 ⋄ else:

  ∧⍵ sort the argument ascending

Keg, 35 bytes

₳0a&᠀:(⑴|⑻-)⒃᠀⒃¬+[Invalid input|¿÷⑭

Nice to see Keg still "up to par" with Golfscript! No regrets about the bad pun

GolfScript, 37 bytes

.$58,48>123,97>+-!*"Invalid input"or

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Explanation

Whoa. Maybe I shouldn't have done this..

.                                     # Make 2 copies of the input,
 $                                    # One is the sorted copy of the input,
  58,48>123,97>+-!                   # Another is whether all of input is in the
                                      # lowercase alphanumeric range
                   *                  # Multiply them:
                                      #     If the input contains invalid characters,
                                      #     e.g. @hog, this multiplies the sorted copy by 0.
                                      # ----If the input is a null string,
                                      #     The input multiplies 0 by the null string, 
                                      #     which is still the null string.
                                      # ----If the input is a valid string,
                                      #     e.g. dragon, the input multiplies the
                                      #     sorted copy by 1 (because everything is
                                      #     inside the range)
                    "Invalid input"or # Do a "logical magic":
                                      #     If the input is 0 or the null string, 
                                      #         returns "Invalid input"
                                      #     Otherwise, this returns the
                                      #         sorted copy of the input.

# Further explanation of the code snippet
#                  # Swap the stack so that an unused copy of the input emerges
#  58,              # Generate a range and drop the last: 0 to ASCII ':'
                    # (The previous character is '9')
#     48>           # Select everything that is larger than 48 (ASCII '0')
#        123,97>    # Same thing with generating 'a' to 'z'
#               +   # Join the inputs together
#                -  # Set subtraction: subtract this string from the fresh unused input
#                   # This results in everything that isn't alphanumeric
#                 ! # Negate the string. If the string has something, 
#                   # it turns to 0. Otherwise it turns to 1.
```

Python 3.8 (pre-release), 77 75 74 88 bytes

import re
print(*re.match('[0-9a-z]+$',i:=input())and sorted(i)or'Invalid input',sep='')

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