The Docker command is not working after restarting (using sudo reboot) the Ubuntu (20.04) server.

Now, for any command with docker, it gives me an error. For example,

$ docker --help
cannot update snap namespace: cannot create symlink in "/etc/docker": existing file in the way
snap-update-ns failed with code 1

When I manually check, there is a file called key.json in the /etc/docker folder which has a json dictionary.

Before restarting, I have had few docker containers running in the background with volume connected. When I run systemctl start docker as mentioned in one StackOverflow answer, I am getting

Failed to start docker.service: Unit docker.service not found.

It would be great at least to recover the docker images that were there before restarting.

-- Edit --

For some reason, docker is working now. I have restarted once again after the initial restart which resulted in the error. But there was no improvement. However, it is working fine now. I do not know what solved the issue, maybe the cmd journalctl -u docker.service (as suggested in a comment) help in some way, or some other reason.

So, It would be great if someone can answer what was the initial reason for the trouble? It might help us to avoid this in the future.