Problematic iPhone 3G - No SIM card accepted (Help!)

[ultimatus]

I live in the UK, and a few years back ~I bought an unlocked iPhone 3G which was going fine until recently when I upgraded its software to iOS 4.2.1, ever since it has had no reception and doesn't accept any SIM card  , it also doesn't seem to be able to be restored to factory settings by neither iTunes nor from the phone itself - shortly said it seems to be blocked once again... Can you please help?
Yours sincerely,

[Transfusion]

You would have to unlock and, in the process, jailbreak your phone, if you upgraded to stock firmware without using one of the methods to preserve your unlock. However, do not fear:
http://osxdaily.com/2010/11/28/how-to-unlock-ios-4-2-1-on-iphone-3g-and-iphone-3gs-with-ultrasn0w/ 
You can unlock again using Ultrasn0w.

[ultimatus]

Thank you very much, I'll try it out and let you know. Thanks again.