Make sure the device is properly charged. When you try the reflashing, first unplug the charger cable (and keep the N800 off), plug in the USB cable, start the PC program, _then_ plug in the charger cable. Then you won't have to do the (sometimes tricky) key combination trick to turn it on: Pluggin in the charger cable will power on the N800 for you.

If the above doesnt work then it sounds like the unit is bad and you should look at getting it replaced/repaired.