because its either broken or its stuck look and find out
There's a few different possibilities:
The speedometer unit itself has failed
The VSS (vehicle speed sensor) that "drives" the speedometer has failed (mechanically driven 1995 and some 1996, electronic since then)
One other circumstance could happen: when there are other failures/symptoms that go along with this: odometer shows all dashes, radio doesn't work, A/C doesn't work, cruise control won't work, and fuse #10 blows as soon as you put a fresh one in.
These are all symptoms of a leaking brake pressure switch.
This is the reason Ford has issued a recall of all Windstars. Check with your dealer for more information about your specific vehicle.