Even if we did our best, sometimes we get unlucky. Maybe the attacker found the vulnerability by himself before the software author get noticed. Or maybe the attacker was simply lucky to find your web site before you had a chance to make an update. Once an incident happens, you need to do the following things.
Firstly, don't panic. Contact us if you require assistance.
You will need to erase all of your contents and then reupload from your local backup. There is a chance that the malicious party inserted a backdoor in your scripts.
Update any program or scripts to the latest version. Contact the software maker of simply visit their web site for more information.
If you happen to know the identity of the irresponsible party, do not under any circumstances get in touch with them. A lot of these people are people with serious mental problems. Talking with them will not make you better, and certainly will not restore your web site. Any contact to these people should be in the form of legal action.