Never heard of that. What I'd check first:

- that the magnet (on the spokes) passes the sensor (on the fork blade) close enough
- that the wire running from the sensor up to the handlebar mount is OK, assuming it's a wired computer
- or, if it's wireless, that the battery in the sensor is OK
- is there dirt/grease/rust/gunk in the small metal connectors in the handlebar mount and/or corresponding connectors in the rear panel of your computer? If there's a faulty connection there, the computer won't work. Again, this is assuming it's a wired comp.
- finally, even though the computer shows time, battery may be nearly dead. You could try changing that too, if everything else fails.

--J