1) Switch off your iPhone.
2) Take your Sim card out from the iPhone.
3) Now insert Sim card to normal Cell phone and make a call,
4) Now Insert that Sim card to your iPhone and Start your iphone, magically it will works.
5) if you still face the same problem then your network not support 3G services check 3G option in Setting --> General --> off 3G service and restart your iPhone this will work.