Phone won't boot after Cyanogenmod 10.2 update

I just updated my HTC One X from Cyanogenmod 10.1.13 to 10.2 RC1, which seemed to go smoothly. It completed the "Checking apps" stage, and when it rebooted, it got stuck on the boot screen (the spinning concentric circles) for about 15 minutes, at which point I held the power button to turn off the phone, but it just restarted and it's stuck in the same place. What are my options? The problem I have at the moment is I can't turn on or off the phone (just restart) and I also can't get adb to work when I connect it to my PC. (oh, and no...I didn't backup before updating - because that would have been a smart thing to do) Thanks for any help.