Doesn't recognize SIM card

Unfortunately, the SIM card reader is attached to the logic board and cannot be replaced independently. You might still be able to troubleshoot this though. First give the phone a reboot if you haven't already. If you you're only having a problem with one SIM card, try cleaning it's contacts with a pencil eraser, as they may just be dirty or corroded. If you're having a problem with multiple SIM cards, try the pencil eraser first, then try a network reset (Located in Settings>General>Reset>Reset Network Settings [in iOS 7]). If this doesn't work, try restoring the phone with iTunes. If you're still having a problem, try taking it to the carrier store and seeing if warranty will cover repairs. Good luck!