Just for curiosity...did you get it wet? Mine quits working whenever it gets wet. In my case, it's somehow related to the sensor. It happens when it's raining, or if I go through a big puddle. I can try drying it with an air hose when I get home, but nothing gets it working again, until it sits overnight. That is consistent. Started happening when it was a couple years old. I can live with it.
There are a couple of things you can check, though. The sensor is just a magnetically actuated switch. All it does it turn "on" and "off". So, if you have a continuity light, you can check the sensor by removing the computer head, and holding your continuity light leads against the contacts in the mount, and spinning the wheel. It should flash the light "on" and "off". If so, the sensor is working.
For the head, you can take a metal object like a penknife, screwdriver, paperclip...whatever...and short between the contacts on the back of the head. Short and release, short and release, etc. The computer should register at least something. If so, the computer is working.