If you recently installed a mod, and you know you did. Try finding another mod and replacing it to ignore a complete reinstall. If that doesn't fix the problem you either to use a backup, or reinstall GTA SA. Make sure you DELETE everything, and reinstall to a new directory if necessary.
I hope that I helped clear up any issues, if you find yourself in some more trouble PM me, also some guides on how to make backups and reinstall/install to a new directory can be found in the GTA Modding Sections.