Look for files in the root of your website. Typically, there is a line added also at top of your index.php file or index.htm. It's expensive to have someone come in and clean a website, about $250.00 each. Do you have backups?
Cleanup is one thing, but it's not enough. You also need to figure out how they got in (weak password? outdated software with vulnerabilities?) and close that door. If you have server logs, it should help immensely. If not, you need to start saving them right away so that you can help recover from future hacks. And by all means, backups will help, but be careful; it's possible that your backups were made after the hack took place -- so restoring them would not do anything for you. And yes, it is expensive to have somebody come in and clean up your website (I get a few of those calls but only do it for really good friends; I dislike getting my hands too terribly dirty!). Best wishes, Tom