Light Locker has known issues of showing a delay while waking up from lock state. The symptoms that you see can be because of this.  The recommended way to fix this is to add an entry for your graphics driver, as shown here in the Debian Wiki entry for light-locker.

However, if that does not work for you, you can try removing Light Locker by using the following command for Debian.

sudo apt remove light-locker

• On debian 10 (buster) I had this problem with lxde/openbox windowing system on an Intel NUC PC.
  • I think it had trouble resuming the graphics as Ctrl-F8 would recover things so chvt program to switch to/from consoles was required to avoid messing up the graphics.
  • This set up used systemd-suspend.service - see man systemd-suspend.service
• My fix was to add the script below that is run before/after suspend/resume from suspend/hibernate (it fixed both).

/lib/systemd/system-sleep/display

#!/bin/sh

case $1 in
   pre)
      /bin/chvt 1
   ;;
   post)
      /bin/chvt 8
   ;;
 esac

I struggled with a similar issue in Debian 9, installed on a Lenovo G40-30 Laptop. I went into Hibernate/Sleep and trying to initiate again the screen didn't show up although everything seemed working.

The solution is actually quite simple. It seems Linux OSs, in particular Debian and Ubuntu need at least a 4+GB swap partition for Hibernate/Sleep to work properly. If you installed with "default" configuration it will create a Swap the same size of your actual RAM (in practice a little less). So if you have a laptop with less or equal to 4 Gb RAM and installed "default" configuration, you are probably trying to solve this issue.

Swap allocation in Linux work in two ways:

1. in the form of a SWAP PARTITION in your hardrive.

2. in the form of a SWAP FILE.

YOU CAN CREATE THE SWAP FILE AS FOLLOWS:

sudo swapon --show 

shows if you have enabled the swap option. If not look up how to do this.

sudo fallocate -l 1G /swapfile

sets the size of the swap you add to 1Gb, change to the value you need.

sudo chmod 600 /swapfile        # sets the file to be owned by root     
sudo mkswap /swapfile           # mkswap tool to allocate swap in the file
sudo swapon /swapfile           # activate the swap 
sudo nano /etc/fstab            # open the file to make changes permanent

Add the line /swapfile swap swap defaults 0 0 to the file /etc/fstab:

sudo swapon --show              # show if its working
sudo free -h                    # show Memory and Swap 

IF YOU WANT TO UNDO CHANGES JUST:

sudo swapoff -v /swapfile

remove the line from /etc/fstab file: /swapfile swap swap defaults 0 0

sudo rm /swapfile         # remove the swap file

SWAP SIZES ACCORDING TO RAM:

I can indicate the following table with some recommended SWAP sizes according to your RAM. Last 3 columns are SWAP spaces:

    RAM       No hibernation    With Hibernation   Maximum

    1GB              1GB                 2GB        2GB
    2GB              1GB                 3GB        4GB
    3GB              2GB                 5GB        6GB
    4GB              2GB                 6GB        8GB
    5GB              2GB                 7GB       10GB
    6GB              2GB                 8GB       12GB
    8GB              3GB                11GB       16GB
   12GB              3GB                15GB       24GB
   16GB              4GB                20GB       32GB
   24GB              5GB                29GB       48GB
   32GB              6GB                38GB       64GB
   64GB              8GB                72GB      128GB
  128GB             11GB               139GB      256GB
  256GB             16GB               272GB      512GB
  512GB             23GB               535GB        1TB
    1TB             32GB              1056GB        2TB
    2TB             46GB              2094GB        4TB
    4TB             64GB              4160GB        8TB
    8TB             91GB              8283GB       16TB

MORE INFORMATION:

you can find thorough information on recommended SWAP sizes according to your RAM in the following link:

How much swap should I take for 1GB to 8TB of RAM on 14.04 or higher?

Credit is due for the table I added here.

I had a similar issue when resuming from hibernate so here's a broad troubleshooting list of things to check:

Does the keyboard work?

Try hitting CAPSLOCK or NUMLOCK and see if the LED goes on and off.

Is your system running?

Put on some music and see if it resumes.

Can you use the computer?

If both questions above answer positively, chances are that you can use your laptop blindly. Start a terminal and run some commands that output sound to check if it is working. If it does, everything is working except the screen not turning on. This is what I had too.

How I solved it:

What might work as a workaround (but probably not in your case) is to close the laptop and reopen it again. Suspending in general helped me out quite a lot.

If not, try calling xrandr.

• You can run it through a terminal if you can use the laptop even without any monitor
• Suspend, wait, and then xrandr: sudo systemctl suspend/hibernate;sleep 60;xrandr.
• Put the command into a systemd service.

Here's mine:

[Unit]
Description=After hibernate
#After=suspend.target
After=hibernate.target
#After=hybrid-sleep.target

[Service]
ExecStart=/usr/bin/xrandr

[Install]
#WantedBy=suspend.target
WantedBy=hibernate.target
#WantedBy=hybrid-sleep.target