Either start from scratch or work hard on fixing them up...
In addition to the links given above, see:
If you decide to start from scratch, then work hard on security to try to prevent this from happening again.
Make sure that you keep as minimal an amount of plugins/themes as possible. If you try something, but deactivate it, don't leave it just deactivated, but actually remove it. Use a plugin like Bulletproof Security as well. Don't install any plugins/themes that use old versions of timthumb.php
Make sure all your software/plugins/themes stay up to date with the latest versions released, most importantly if they note any security additions with the new release.