Chandu, try reset the network settings with sim ejected and wait till phone says no sim then insert again. Usually that should fix the issues if it has a sim pin also. If stuck in iTunes, do you know if it is displaying the baseband information etc? Or is that blank? If it's blank, the only thing I could suggest is to restore the device once more, otherwise try a reflow of the chip itself if your comfortable with the idea, or get someone who you trust with electronics to do for you.

Plug the device in without the sim card, and go to Settings -> General -> Reset -> Reset All Network Settings. Let the device reboot, and once complete and no sim is displayed, you can put your sim card back in and all should be fine.

Unfortunately, I have found that it seems the comm board and logic board seem to be "paired" up... I can't understand why or what and a fix would be great, but what I DO know is that paired logic/comm boards work perfectly, whilst around 10 units I've had don't work with just a new board or just a new comm board.... Weird I know, but so far I've only gotten them working with matching boards...

Also I would like to add, that 9/10 the solution is very simple. Remove the two dock screws and it generally works. Unless you dropped in water or something stupid like that. I replace and repair items for a living, and I came across this when our first unit with dock problems came in. I believe it is a manufacturers defect that causes the problem, someone can correct me if I'm wrong, but for whatever reason, removing those two screws that hold the dock connector in place generally solve the problem. I joined this community to help and now I think genuinely I found an answer that will help a lot of people out.