I have not had that issue (yet) on my P800, but I did have it once on my old 3800. The front lid was held in place by a small magnet that had come loose from its socket. I simply glued the magnet back in place and it never had that problem again. I wouldn't have been so daring if the failure were in a part that was essential to printing.
My P800 has what looks like a tiny magnet in a movable holder attached to the top of the printer case. When you close the front feed tray, this "magnet" fits into a socket on the lid. Perhaps the magnet-like thing has detached on yours?
If I have time tomorrow, I'll try to photograph the presumptive magnet. and post it.