Restart MySQL

service mysqld start

if using MariaDB use

sudo systemctl start mariadb

@Mohsen47

Dear, When i will enter this command on PuTTy, i get an error
tail: cannot open ‘/var/log/mysql/error.log’ for reading: No such file or directory
Note: I have 512 MB Ram Server with lot of Bandwidth Available. I Will Deploy Server and Start Installing WordPress, everything was fine, but when i will try to install theme, after this it shows an database error.

@salmanafzal

The error message means that mysql process has crashed for some reason please share your mysql error log by giving as the output of this command

tail -20 /var/log/mysql/error.log

My guess is that your droplet is running out of memory how much memory did you allocate for it?

Hope this helps.

lsof -i:3306
kill the running process with
kill -9 PID
then restart the mysql service