The tach's are run by the voltage regulator. The voltage regulator is water cooled. If you have a voltmeter, look and verify that is is working and not just reading battery voltage, is it charging? If not, the regulator is bad.
If you have recently overheated, this will burn out the voltage regulator which will cause the tach to quit working.
I'm thinking its simply a bad tach. It worked fine one trip and then the next time I took it out (about 4 weeks later) it was stuck. I was able to tap on the glass and get it to move, but now it won't even do that. Called the place I bought it and its under warranty, so I'll be having it replaced this week.