Stolen iPhone now recovered but id locked
I purchased an iPhone 4 in June 2013. The device got stolen on 3rd May 2015. I filled FIR with law enforcement. Fortunately today, ie 4 months later they recovered the device and handed me back. But i can't restore it to factory defaults, since the thief have used a different apple id and it asks for password to that id before resetting the device. I have all original documents relating to purchase and the phone was purchased in my name. Kindly help!
iPhone 4