See the thread:
Yikes, I've been hacked! Now what? (http://forum.coppermine-gallery.net/index.php/topic,51927.0.html)
It was written a while ago, but the basic steps still apply today.
- Read through the thread completely, as you will need to follow all of it
- Make backups
- Try asking your hosting company for some help, they might have a backup of your site