There's a couple of ways to reset your N800 in the instruction PDF at the Nokia site. Being that you have no Internet, I would try these. There's a "soft" reset (PDA terms) and a "hard" reset. I don't know how hard the hard reset is, but if my device couldn't connect to the net, I'd definitely try that.
Also, I wonder if theres a way to put a image on an SD card and re-flash the N800 from that? Hmmm.
Alan