If you have a backup, restore it. If not, see if your host has a backup and ask them to restore it. You also need to make sure you are running the latest versions of the forum software. If you are on a dedicated server then you will need to work out how they got in. If not then look through your logs and see if the script was vulnerable.
Your host should have a weekly backup. Just ask them to restore the last version. They may charge a couple bucks though...
i have hasty host account by the way i delete my forums from login my Cpanel Hackers hit index.php file