My hour meter did the same thing a year before I rebuild the motor. If the Tach is still working, you can do what I did at the rebuild. I added a Hour meter like the one below. Just cut a 2-1/16 inch hole in the white section of the recess, install meter. You can also pull the whole recess out. If you want the hours of the new meter to match the actual hours, just hook the meter to a 12VDC battery for X amount of hours. I actually replaced all my meters during the rebuild except the Tach. The hour meter needs to be waterproof they cost about $11 to $40. Replacement tach w/hour meter is about $150.
http://www.amazon.com/Sierra-International-56966P-Round-Marine/dp/B0014477MU/ref=sr_1_1?ie=UTF8&qid=1342663726&sr=8-1&keywords=marine+hour+meter