un-jailbreak iphone 3gs ( version 5.1.1 )

[viosteaua98]

hi all ,
I have an Iphone 3gs , recently i jailbreaked it to 5.1.1 with redsnow 0.9.11b4, after i installed 5.1.1 i saw my phone is not turning on without boot by pc ( redsnow ) .
Now i want my original soft back or i want to un - jailbreake it.
I tried to restore it by Itunes but when i try to activate the iphone it is saying i have no signal ( SIM ) and i need it to activate the iphone
How can i un-jailbreak my 3gs to orignal soft or to a new soft but not jail breaked