[Frosty]

As Lynne said, check access_logs, they (he) could have uploaded a php shell, which allows editing of all files that are writeable, and some shells have the ability of altering the MySQL database, which could explain why your password was changed.

If that isn't the case, scan your PC with Malware Bytes and/or Spybot, your PC might have been infected by a keylogger or a similar program that could give out your passwords to the attacker.