[CarlitoBrigante]

You got all good suggestions here. We worked on stuff like this a lot of times, and, in order, most of the times the issues were caused:

1 - Installed scripts forgotten by the owner, like phpmyadmin and/or old outdated mods
2 - 777 directories in shared environment - if this is a possibility, report it to the hosting provider
3 - Bad DB password in shared environment
4 - Fully compromised system. Somebody got your root access in some way and can do whatever he wants on your server until you reinstall everything and overwrite possible backdoors
5 - Very rarely, guy hired by the webmaster in the past having fun thanks to unchanged passwords. But keep all your login passwords fresh, rotate them as frequently as possible