87 Fleetwood Cadilac Sixty special

Take the car to a rebuttable garage or GM repair facility and have them scan your car's computer for diagnostic trouble codes (DTCs). Their scanner will be able tell if the problem in your digital display (assuming you have one), or if the problem is in the power-train.

If you do not have a digital display, then the problem is more than likely that the speedometer cable, on the back of the speedometer, has come off.