In quiet room, move the magnet back and forth past the sensor. You should hear it 'click' softly as the magnet passes. If it does, re-check batteries, then if it still has no display, it may be time for a new unit.
BTW I used a wireless until I found out it would not work while my Niterider MOAB HID headlight was on. Switched to an Astrale 8 with cadence, and have had no issues since.
Thanks for the advice. I could hear the click, but even when I reinstalled the battery, nothing would register. I also reset the computer unit and reentered the data.
For the past couple of days, I've used a Garmin GPS unit instead of a cycle computer. It's worked, but it also has its drawbacks. (Starting it up takes a little longer, the odometer doesn't register cumulative totals after I shut it down and then restart it and I'm constantly recharging the batteries.)
Tomorrow or the next day I'll be at the bike shop, preparing to replace the computer. I'm hoping the next bike computer will last me longer than this one has lasted.
Trek 730, Bike Friday NWT, Brompton M6R, Trek 830, Dahon HAT 060, ...
A wired computer appears to react to the change in contact between its terminals, with the sensor containing a reed switch that is closed by the magnet. As a further test in trying to figure out what the problem is with your computer, you could take a piece of wire, maybe even an unfolded paper clip or semi-open scissors, and repeatedly connect and disconnect the computer terminals. The computer should react to those as if the bike were moving.
The only "other thing" is that sometime there is an "all-clear" procedure that must be done if a battery change results in sort "freezing" the computer program. Sometimes its a little tab of metal near the battery holder inside.
You really need the owners manual to know if it exists.
Yesterday I brought the bike to the bike shop. It turned out the sensor unit had become defective. This was replaced and now it's working nicely once again. I'm happy I didn't have to replace the entire unit.