Can't connect to MySQL! "Connection for controluser as defined in your configuration failed."

I have a PHP application written with codeigniter that connects to MySQL just fine for two months. Yesterday, it couldn’t connect to it, showing an error message that there was an error in the configuration or something! I did not change any settings at that time… I tried to log in with phpmyadmin it told me this message “Connection for controluser as defined in your configuration failed”. So, now I know that neither me nor the application can connect to the MySQL server. Restarted the droplet and everything got back to normal! Today, it did the same thing again -_-, just made a reboot and its fine again…

My question is: What is the real problem so I can prevent this from happening again?

PS: I kinda noticed a new database now within MySQL i am not sure if it was “mysql” or “phpmyadmin” but i am sure they both were not in there… I use the root username. Droplet runs Ubuntu.

Thanks in advance…