Iphone/ipad/imac erased by hacker using 'find my device' and in process they locked my iMac with pin code! Anyway to get around this?

Last night I suddenly found on my iphone and ipad screens a message from 'find my ipad/iphone' that they were lost and to call my phone number. When I entered my requested Apple ID and password they had been erased. I was able to restore with icloud. However, not realizing what was happening I turned on my iMac. Login screen appeared and before I could do anything the computer rebooted to white screen requesting the 'system pin code' used to erase and lock my imac that I set up through 'find my imac'. And an ugly message left by the hacker. I also found the person had bought a new ipad from the Apple store using my ID. I changed my Apple ID password and removed the credit card tied to my account and cancelled it. How can I solve the Pin code problem? Is my drive in fact erased requiring 3rd party software to recover my data? Could I use the recovery partition or a recovery/install thumb drive to try to reset the firmware password? Would target disk mode help me get my data back? Could I use terminal in recovery mode to override or reset the pin code? Would any of these attempts make the problem worse or have me lose my data? Or is it already gone and now I just need to get access? Any help would be greatly appreciated!

iMac mid 2010

OS X Mavericks

Have a recovery/install thumb drive with mavericks and snow leopard install disks that came with the imac

No time machine backup

Manual backup of important files, recent but not as recent as I would like.