Original post by: JackoAllMasteroNone ,
Have you tried re-seating your sim card? I have seen these get loose and just not make a good connection with the phone. Take the sim card out, make sure there is no dirt or debree like lint or anything on the card contacts or in the slot in your phone. Re-insert card and make sure it is in there secure. If it is just a loose sim card and it continues to happen try applying a piece of tape or something thin to build it up just enough so the back case puts enough pressure to hold the card in place. Hope this helps!