Your questions is quite generic but I’ll try and lend you some pointers on which you can start.
Check Disk Space
If your droplet is full, services like mysql,apache,nginx etc. will start failing as they can’t create the proper sockets to start themselves.
Check the current disk usage
That will show you all your partions and the size they are using. If your server is full let me know and I’ll provide you with pointers on what to check and clear.
Now onto the good stuff
*Your MySQL might have been corrupted. *
In order to check that, you can check your mysql error log. Your error log should be defined in your my.cnf file. If you are not sure where it is, you can do the following
It’s usually at /etc/my.cnf
Once you find it, open it with your favourable editor and see if there is a line like
log_error = /var/log/mysql/error.log
If there isn’t try adding the line and restarting MySQL.
Once you do, the error log should appear and it should say what the problem is.
If you run into trouble, post the contents of it and I’ll help out.