What happens if my car was driven by someone without my permission and he gets in an accident?

If they took your car without you permission, it is a vehicle theft. You would not be civily liable for an accident. Where the waters get muddied however, is if that person has driven the car with your permission in the past. He could use that as a defense in a criminal matter, because he could say that the permission to use the car "stands". It is doubtful you would incur any liabilty.