ICQ>

Sounds like the little hairspring inside has broken. The tach is probably toast, although a repair service may revive it. I once soldered a broken spring on a similar meter movement (not a tach, but the same inside) and it worked for years. There are several instrument repair services that advertise in the classified section of Rolling.

Unless you're comfortable (and skilled) soldering microscopic stuff, I'd leave it for a repair shop to fix.