IT DEPENDS, SOME WORK BY GEAR AND CABLE SYSTEMS FROM THE TRANSMISSION. OTHERS CAN BY MAGNETIC OR COMPLETLY ELECTRONIC.
There might be a problem with the cable.
When you step on the pedal, the motor is propelled by a ticker which forces the belt to turn. This then propels the wheels forwards and the speed on the speedometer is determined by a mechanical...
its likely to be a fuse problem. the fuses can be found underneath the steering column usually
check your transmission and gear so test if there ok if thay are dont drive your car for a while if its bad got to a shop hope i helped :)