help please Iphone 3g 4.2 upgrade, emergency call only

New to this unlocking and jailbreaking thing, so please make it simple as possible for me. I had an unlocked iphone 3g, upgraded to 4.2 and now im stuck in emergency calls only with itunes showing. The original phone was with at&t, now im with another carrier. The phone was working fine with the other carrier, i just wanted to update software. I dont have an at&t sim card, but could borrow one from someone. Is that what I need to do? And if so, could you please explain the steps, including any software i need to download first on my laptop. Please go simple. Thanks so much with any help.

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