Mongo exception: connect failed on reboot

As MongoDB BOL Here The MongoDB instance stores its data files in /var/lib/mongodb and its log files in /var/log/mongodb by default, and runs using the mongodb user account. You can specify alternate log and data file directories in /etc/mongod.conf.

If you change the user that runs the MongoDB process, you must modify the access control rights to the /var/lib/mongodb and /var/log/mongodb directories to give this user access to these directories.

Issue the following command to start mongod:

sudo service mongod start

For your further ref Configuration File Options

mongod is actual mongodb daemon what "is" the database. mongo is just client program what connects to mongod process (in this case; with sharded cluster it connects to mongos -process).

So, mongod process must be running before mongo can connect it.

You wrote that sudo systemctl enable mongod.service don't work anymore? Check if that service has (some how) changed its name. Just give command sudo systemctl|grep -i mongo and you can see if it is there and it is enabled. IF still your mongod process don't start automatically after reboot, check /var/log/mongodb/mongodb.log (you can check that lock file name with sudo grep path /etc/mongod.conf -command) says. What is the error? Fix that error (maybe chown of directory and files?) and try again with sudo systemctl restart mongod.service (or what ever is services name)