My iPhone says no SIM inserted?

I dropped my iphone 4s on the ground, it seemed fine but then when i looked at it, on the screen it said no SIM inserted even though my SIM card is in it, what do i do? Thanks

Added (1). I have also tried everything, i, turned it on and off, i took the SIM card out and back in, and i reset the settings about 5 times

The sim reader probably got moved when it was dropped.
if it was never jailbroke, no water damage and still under warrenty apple will fix free.