Sounds like it could be one of two things. You will need to remove the buttstock to get access to the safety. Check to make sure that the safety spring is still in the guide pin on the upper receiver and that the spring is still providing tension to the safety. Second thing and one I have encountered is that the pin in the lower leg of the safety that provides tension to the safety has fallen out if it has you may have to sacrifice a cheap drill bit of the appropriate size to make a replacement pin .
