My websites are not working as database is not connecting because MySql not running.
On restarting MySql I got this error

sudo service mysql restart
stop: Unknown instance:
start: Job failed to start

Earlier I updated some WordPress plugin on one of the site.
How to resolve it?

Your MySQL log should give you a better idea about what is going on. Try looking for /var/log/mysql/error.log or /var/log/mysql.err

Though updating your Wordpress site may cause a problem with the site's ability to connect to MySQL it shouldn't prevent MySQL from restarting properly (shouldn't, but I guess it still could).


MSJ

A full disk often causes this what's the output of

df -h

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