Well, I am not sure if it's going to work but still let's give it a try. Go to My Documents > GTA:SA User Files > Backup your current gta_sa.set. Now as you have the backup, delete the gta_sa.set (Not permanently) from the user files. This will restore your default settings for screen resolution and stuff. Try launching the game, if it doesn't work, replace the gta_sa.set (which will be created automatically again after launching the game) with the existing one in GTA:SA userfile. The backup file contains all your previous settings, which will be re-gained after you move it to user files again (If the deleting of existing one doesn't work for MTA:SA).