It's basically a manifold pressure gauge. If electric there's a sensor screwed in the intake manifold downstream of the turbo and a wire that gos to the gauge. The sensor wire could be disconnected or broken, or you could have no power at the gauge.
If not electrical, it's a pressure gauge with a piece of tubing running fron the intake manifold to the gauge. If so, the most likely cause is a break in the tubing.
I haven't explored my 1996 Sahara enough to know whether this if electric or pressure (Brett Wolfe?)