Can't receive any networks after unlock

Hello,

A few hours ago I purchased a Samsung SGH-J700V prepaid phone from Vodafone. My last phone broke down yesterday, so I thought I'd buy a prepaid phone and have the simlock removed so I can put my own sim-card (also Vodafone) back. However, when the guy from the unlock-shop unlocked my phone and put in my sim-card it said it could'nt find any networks. We tried the original prepaid sim-card I found no networks either. So I tried a few other sim-cards from and neither of them worked. I am afraid the dude at the unlock-shop broke the phone. Is this a known problem after unlocking? Does this mean I can throw the phone in the trash? Or is there some way to do a total reset or something so it can find networks again? I don't even mind my own sim-card not working. If I can get the prepaid card to work I'd be pretty happy too. Please help... I'm getting pretty desperate (and frustrated)