My guess is a protection circuit was triggered and needs to be reset. I read the following post on another forum, which I have copied below:
"I saw this problem from a Sony ES receiver after I...ahem...shorted out one of the speaker leads while it was playing. It would start turning on, and then when the relays would normally kick on the speakers, it would shut down flashing the Protected message. There was a 'factory' reset procedure that a tech walked me through, but it didn't work. I can't remember the key sequence either."
While it doesn't provide an actual answer to your question, this quote suggests you should call a Sony tech to try the reset procedure. Or, perhaps it wouldn't be so expensive to take it into a service lab if all they need to do is walk through the appropriate reset procedure. (Maybe somebody on Audiogon knows the procedure?) Good luck.