On most Fords, the cruise control won't work past 85 mph. I'm not sure if the Contour is set up like this or not. However if your cruise doesn't work (and it was before this happened), that would be a good indicator that the speed sensor or its wiring is faulty. You might want to try scanning the computer for codes too. It might be smart enough to know that there's no way you can be doing 120 mph from the moment the engine starts, at idle, with the throttle closed. In that case it should set some kind of "VSS out of range" code.
If either of those apply, I'd grab a cluster from a junkyard and cross my fingers.
A Ford dealership could also diagnose it by reading the VSS signal directly from the computer, but be prepared to pay out the ass for someone to spend 3 minutes looking at a scan tool.
