Sounds like a problem with the Z sustain board.
But the fact that you still have sound, it's more likely the Y sustain board.
As for not powering off...Unplug it from the mains and try a reboot.
Does it respond to the remote?
Both the boards should have fuses, so you could check those first.
Be careful though, don't touch anything else as there are high voltages inside.
Martin.