I'm assuming you have a 94-98 car from your avatar. If you have a 99-04 or a trans out of a 99-04 then you don't have VSS.
Well first take a peek at the VSS gear on the VSS sensor in the tailshaft of the trans. It is easy to get to and is a 8mm bolt IIRC on the left side of the tailshaft. Removing it will not leak any fluid so don't worry about that. You are inspecting the gear on the sensor and also shining a light in the hole and making sure the teeth on the driven gear are there.
If all that is kosher you can check to see if you have any broken wires or if you are running a speedometer recalibrator box you can try bypassing it to see if it is the problem. Finally, there is the speedometer itself. These are electronic speedometers and the motors are known to fail in them, would not be the first time. The good news is a cluster is a 15 minute swap, the bad news is that PATS may hate you if you are a 96-98. I've you've got a tuner you can just have PATS disabled though.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.