No Sim Card message in my iPhone 4 after upgrading to ios-6 please help how to resolve this issue

Hi

I have an iphone4 which was working fine all these days until i upgraded to ios-6 from ios-5.

No i see a msg in the top left corner as "No Sim" and it also poped up that msg when i had completed the upgrade.

I had completed the 2 years contract with ATT and had got it unlocked with ATT help.. Can someone please tell me how to resolve this issue.. without my phone i feel lost..

Thanks

Vishal