If loose wires and connectors are not obvious, open the pedestal panel to gain access to the tach backside and try rotating the potentiometer back and forth. Our units have  tendency for some internal corrosion. Be sure to reset it to the same position or the tach calibration will be off.

Also, make sure your alternator is putting out.  Tachometers get pulses from the alternator.

Please consider adding your boat model, year, engine, non-standard add-ons, etc. to your signature.

I also had a problem that was solved when I checked the wire from the alternator and found the wire disconnected at a terminal block . Looks like the engine cover was the culprit when it was put on after an oil check.
Good luck