Give me more information and see if I can come up with a reason:
1. Which part of the dash you bang on to try to fix it? On top of the dash board? on the plastic lense?
2. When you say "speedometer not working", is the needle staying at zero mile/hour, or the mileage reading not incrementing, or both?
3. If it is the needle that is not working, is the trip meter also not working at the same time?
4. Does it happen to you that the needle goes back to zero mile/hour while the car is in motion going forward?