im not a genious when it comes to the intracite workings of cars (love cars and learning more and more by the day) but obviouslt the T-Stat cant be shot or you would know that...the sending unit has to be crapped out or its unplugged from the back of the cluster...
there is really only a couple of things, it wouldnt be the thermostat, that could mess up the engine temp which would show up on the gauge but would not leave the gauge dead
the sending unit gone bad
the gauge itself gone bad
wires broken or bad fuse
its most commenly the sending unit but check wires/fuses first because thats free.