Sounds like you got hacked. Work your way through these resources and follow all instructions to completely clean your site or you may be hacked again. See FAQ: My site was hacked « WordPress Codex and How to completely clean your hacked wordpress installation and How to find a backdoor in a hacked WordPress and Hardening WordPress « WordPress Codex.
Change all passwords. Scan your own PC. Use http://sitecheck.sucuri.net/ before and after.
Some plugins might be useful: Search for "security" « WordPress Plugins
Tell your web host you got hacked; and consider changing to a more secure host: Recommended WordPress Web Hosting
If you can't do the work yourself, consider looking for a reputable person to fix it correctly on freelancing sites such as Elance. (It's not a good idea to respond to unsolicited emails from forum users offering to work for you.)