an app you can download from the net called "ziphone" can unblock the networks to dual band an iphone as i done this myself to dual band mine, not 100% sure if it will get it back online and i think at £275, it is well cheap so would have probably been stolen as they can fetch a lot more than this off ebay.
if i was you i'd report it to the police as if he's done it to you, you wont be the first or the last and maybe they can trace who the phone belonged to or who had it that the phone companies keep a track of (only police can get this info from the network provider).