Welcome to the Support Communities!
The first thing you can try is to turn off the iPhone, remove the SIM card and reseat it in the iPhone.
If reseatting the SIM card and restarting the iPhone does not resolve your issue, the link below will direct you to your wireless carrier for support.
Sir, i successfull restored my iphone 3gs( ios ) from 4.1 to 6.1.3 by itunes, now its asking for activation. when im going for activation its saying that no sim card is inserted but i have vodafone sim inside my phone... but it not covering signal. so im using my wifi to activate my iphone but its not activating... then i used itunes to activate by it's again saying that no sim card detected ... HELP ME PLEASE... im stuck here..... i restored my iphone after resolution of many errors...like 16xx, 30xx, 10xx and many more now i have done that much just activation is remaining so please help me .... you are my last option please