About once a year mine stops working. The electrical contacts where the unit attaches to the mount get dirty. I remove the unit from the mount, clean both contacts on the unit and both contacts on the mount with a Q-tip, and then it works again for another year. I moisten the Q-tip in rubbing alcohol but a dry Q-tip might work as well.
1. Sensor alignment. Has the sensor shifted or rotated? Is that little line still lined up with the magnet? Can you move the sensor so it's closer to the wheel (without the magnet hitting it)?
In addition to JPMacG's suggestion, take a pencil eraser to the contacts to "erase" any corrosion.