Speedometer rests on 100...

Is your gauge shot?
Not necessarily,re-read BadSports post
The cable could distort the cage enough to misalign the guts, try his advice and see if it drops. Or just disconnect the cable and see what happens.
If it still remains stuck, smack the top of the dashboard with your palm. If it still remains stuck...... and you cannot see the needle stuck on the dial or the clear face pressed down onto the needle, then I imagine you will have to remove the cluster for a look-see.
I have dinked around with those cages and tightened up the bearings in the past, with good results. Your results may vary. They are delicate, but not rocket-science.

Will a new cable fix this? I doubt it, and don't see how it could. A dry or kinked cable may wind up and then unload, making your needle jump up and down. So will a damaged drive pinion inside the tailhouse. But the cable cannot lock the needle to a new rest point...... unless as BadSport indicates.