Well, the tach drive off the engine (the part the cable slides into) could have failed but it really shouldn't.
Pull the tach cable from the engine side, note the position of the drive and then turn the prop by hand. The drive should rotate with the engine. If it's hard to see, you should be able to feel it turn when the prop is rotated. Of course if the drive doesn't rotate, well, it's time to talk to the A&P. You may be able to cap the existing system and convert to an electronic tach.
One thing that happened to me when troubleshooting my tach problem was one time I assembled the tach drive at the engine side and didn't ensure full engagement. There is a right angle drive on mine and there's just enough play in the system that the drive can look fine externally but the internal shaft isn't engaged properly.
The easy way to test engagement is to loosen the tach cable from the tach and see if the cable rotates by hand. It shouldn't of course since the engine isn't turning and can't be driven by the tach.:smile: