check the fuse panel. there are only 26 or so fuses. pull them all out one at a time and make sure their not blown. Is there anything else electrical thats not working?
Obviuosly power is not getting to the tach box. The main plug for it is on the right side of the steering shaft that snaps in place below the tach box, you must take apart your lower paneling of about 6 screws to see it. Maybe, jiggling it would fix it but take it off and put back of after seeing the contacts.