hacked again and again

We host number of websites in site5.com developed for our clients and been hacked repeatedly last couple days.
I noticed that I received the reset password message by someone before our sites are compromised so I was wondering
if it had something to do with it. The whole files were deleted and replaced by index.html with some colorful hacking messages.

We still don’t know what was the cause yet but scanned all our computers to see if there are username/password were stole from
malwares but can’t find anything there.

Let me know if any of you have idea what is going on.