There is a square driver that is inserted into the end of the drive shaft and drives the speedometer adapter. You could have broken the insert (cheap, a pain sometimes to get the broken part out of the drive shaft), broken the cable, frozen the gear in the speedometer drive adapter, or...
It's VERY common when the driveline bearing starts to fail, it starts to drop, ever so slightly, which causes the square driver to shear. You think it's just the driver, but it's the bearing going bad that is the problem. Consider any speedometer failure in mid-90's arctic cats to be driveline bearing failures until proved otherwise!! Check that bearing, pronto!!
I have had 3 of my 4 cats do exactly that; speedometer stops, after much checking and replacing the cheap stuff, find a bad PTO bearing on the driveline.