Currently Being ModeratedApr 22, 2012 7:13 PM (in response to LUMIA)
If your phone wasn't officially unlocked and you recently updated the iOS, the updat relocked the phone to the original carrier and you will have to contact the carrier it is locked to about unlocking it. If that is not what happened, then try turning the phone off (hold the on/off button until your see the off slider, slide to turn off), remove and reinsert the SIM (see http://support.apple.com/kb/HT5163), then turn it back on.
Currently Being ModeratedApr 22, 2012 7:22 PM (in response to randers4)
The iPhone 4S cannot be unofficially unlocked. The 4S has a known issue with the sim card. Try resetting the iPhone and reinstalling the sim. If it still does not work go to your carrier and pick up a new sim.