Your tachometer receives information form either the alternator or a dedicated tachometer sender on your engine block near the flywheel.
If you have an externally regulated alternator the tachometer receives information from the alternator. Look on the back of the alternator for the field and stator wires. They are usually blue and white. If they are there check their condition for corrosion
and continuity. (wires can break inside the plastic connectors).
If you do not have the blue and white wires at the back of the alternator, look for a pair of wires that are orange and blue/red (Yanmar colour code). They will be attached to the tachometer sender. You may have knocked one loose.