Firstly, you can go to WHM Panel and search for "Service Manager" and select MOnitor option near by MYSQL.
If you do not have a WHM panel then go to SSH
Run this :
and add this code to the file
* * * * * /sbin/service mysql status || service mysql start
this checks every minute MYSQL status and if result is false then run service mysql start command.
Do not forget to save the file.