Even if you are really up on security for your website it is possible that your site could be hacked, if the server your site is on has been compromised. So, the first thing you do is panic but then what are the next steps?
The first thing I recommend doing is contacting your web host to see if they will assist with the issue. Some hosts will restore your site from a backup, others will help for a fee, and others will say “it’s your problem, not ours.