I would bet you either lost a ground or your tach is dead. I'm leaning towards the latter though. You can pick up a tach at a junk yard for about $50. One from an LX, EX or DX should work in your car.

If you feel ambitious, you can check for loose grounds under the dash. I don't know where they are right off hand. You will have to find a service manual somewhere.(Haynes are cheapest, but they don't offer a whole lot of detail, but they will get you by.) You can get them at any auto parts store for $10-$20.

There are also various places that rebuild tachs, but the cost is very expensive.(between $200- $400)

Good luck.