Database connection error (2): Could not connect to MySQL. on all my websites

i have installed ispconfig 3 on digitalocean debian vps with 512 ram for 3moths now. everything was working well but now i cannot log into ispconfig as i get "user or password empty", also i cannot access any of my wesites on the server ( 3 joomla websites) - i get "Database connection error (2): Could not connect to MySQL." when i try to navigate them from browser. but if i restart the vps server, everything works fine for about 10 minutes then the issue strats all over again.

please help.

 

Your mysql-server stop working for some reasons. Check the mysql-log in /var/log

 

Thanks Florian. i just checked /var/log. i got mysql folder, mysql.log, mysql.err, mysql.log.1.gz. but mysql.log is blank when i check with vim and mysql folder has no files. i am a newbie

 

I think your SQL has problems. A complex SQL query can make full memory and turnoff mysql service