Unlocking iMac when locked by Find My iPhone?

My iMac 10.8.2 was stolen recently during a burglary. I immediately used the Find My iPhone app on my iPhone to locate the iMac and lock it as soon as it was connected to the internet. Within a few hours, it was located and the police recovered it and made two arrests. The individual who was caught in possession apparently told the police that, as soon as he had started to use the iMac, he received my Find My iPhone message that the computer was stolen and the screen went blank. I now have the iMac back but, when I start it up, I get a grey (gray) screen with a little progress bar towards the bottom. When this bar get about a third of the way through, the iMac switches off. Starting up while holding the option key, gets a similar grey screen but with options to chose the boot disc and connect to a network. I can connect to my home network but, selecting the iMac hard drive again results in a switch off. Selecting the recovery disc allows me to bring up a Safari browser and the Disc Utility but I cannot seem to get any further. Using the Find My iPhone app from my iPhone allows me to send a sound to the iMac and I have an option to Lock or Erase the iMac, and nothing about unlocking it. I don't know if something was done to the computer after the burglary and before it became locked but I don't think so because there would have been little time to do so. Any suggestions as to how to unlock (if that is the problem) my iMac?