I've run into this issue today on Ubuntu 18.04 using 4.2.0 mongod CE:

1. `sudo service mongod start` fails
2. looking into the log `/var/log/mongodb/mongod.log` shows that it failed to start because another process is listening (Socket in use) error code 48
3. doing a `ps wuax | grep mongo` I can see that a process of mongod is already running as `mongod --bind_ip_all` user 999
4. killing it with `kill -9 $PID` doesn't do anything, the process respawns. using pkill or htop to send different kill commands also fail, the process stays there/respawns with new pid.
5. terminating it with `sudo service mongod stop` doesn't do anything, it still stays running
6. connecting to it via mongo cli simply hangs-up at the connecting part.

This happened without any abnormal termination or closure of the platform.

Short of uninstalling and re-installing, I have no other way to use mongod at this point?