Describe what u did! What u mean broken?! If u had a jailbroken/unlocked phone by software and updated via iTunes, u got ur baseband altered and locked it! Is that it? Can't downgrade baseband, since Apple erases ur older files.
In these cases, the trick is to upgrade outside iTunes, so you don't get your baseband altered!
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I used to be in IOS 4.1 and upgraded to 5.1.1 preserving baseband. Then unlocked using SAM unlock, cause i was for sometime using a GEVEY SIMCARD PRO. Not anymore...
American iPhone4, "locked" to AT&T. Using it at Rio de Janeiro, Brazil, at Oi local company.