In my experience when the tank monitor's go awry they almost always read too full--not empty.
Does any of the LEDs light? (Even when empty we have at least 1 LED on for "E") I would look around for a disconnected wire between the tanks and the panel (start at the tanks, perhaps the bottom wire fell off). Although since none of the tanks read proper except propane it may be more of a problem at the panel--you may have to pull it off the wall and look for missing wires.
If you read through this thread: http://www.thorforums.com/forums/f27...nsor-1177.html
you'll see how the tank sensors work which may give you some hints on where to look next.