dunno your make or model, but if its not a blown fuse, its a sensor. check for fuses under the dash that are blown, if not that then check for an engine light, and/or take it to autozone and have your codes checked.
Older vehicles use a cable-driven speedometer. Most late model cars use sensors in the transmission and/or drive train. With these systems, signals from the sensors are processed by a module and displayed on your instrument panel. Usually all of you electrically powered gauges will be on one fuse, so if the others are working that's probably not it. Your most likely culprit is a vehicle speed sensor (vss).