My speedometer isn't working what reasons is there

could either be a faulty gear in the speedometer system, speed sensor may be faulty or an ECU issue, try the sensor first and see if you have any luck then see if you can get anyone to fix or fit a new instrument cluster. hopefully its one of those 2.

Have you recently had any work done with the stereo or the AC unit that requires the center of the dash to be removed? There is a computer behind this section that, when it is removed, can affect the Tachometer from working. When I was fixing this issue myself, I was told that there are occasions where a coin or a random metal item can fall into the vent cracks of the module and short it out. That module behind the dash could be the problem.