Unplug the receiver from the wall. In fact move it from the room and plug it in w/o any speakers connections etc in another room and see what happens.
I had the same problem with my Onkyo 573 where all it would do was click. Apparently the circuit was overloaded. Ran an extension cord from an outlet on a different circuit and all was fine. Within the next few days we had a transformer blow and the utility company had to come out and fix the pole. Plugged the receiver back in to the original outlet and ever since then everything has been working fine.
This just happened to my TX-SR806. I've unplugged everything from it, but it still just sits there and clicks. If you do a google search there are a lot of forums which say that it needs to be brought in for service, but none of them have an update as to what the problem was and how much it cost to fix. Anyone?