Wow, sorry for being so way late in replying to this thread.
I had mine do this same thing once upon a time and freaked out as well. I ended up fighting with flashing a .sbf file using RSD Lite; took a couple of different computers to finally get the flash to take.
But at the end of the day, I also read that pulling the battery for about 30 seconds then reinserting it and powering the phone on should get you through. Unless of course, the battery is close to dead. That is where a battery only charger comes in handy.
Hope this might help any readers who come along later . . . .