Broken speedometer on 99 jeep grand Cherokee. ?

So I have a 99 jeep grand Cherokee with 170,000 miles on it. The speedometer gauges have done something wierd, the arrow thing (sorry) goes all the way around the gauge and remains still under the peg. I haven't been able to trouble shoot it, so I'm wondering what could be wrong with it. It remains there whether the car is on or off. All other gauges are fine, just the mph gauge and rpm gauge.