iPhone 3GS went to emergency calls only screen for no reason

My iPhone 3GS went to the emergency calls only screen for no apparent reason earlier. When I tried to restore factory settings in itunes it says there's no sim card installed. I was using the phone earlier today and all was well. Is there anything, short of going to the apple store, that I can do to remedy this? I've yet to speak with someone at at&t customer service, but I have little faith of finding a solution there.