Could use some advice (Unlocking bad imei device)

So i recently purchased a used Optimus G from craigslist, the deal seemed too good, the phone worked great had no scratches and had always been used in conjunction with a case. Few days later the phone was reported stolen and my sim card was locked. After calling At&t I've found out that my LG was stolen and can no longer be used with any sim. I was wondering if i unlocked it through an unlocking site if it would work again, and in that case whats a good service for that?