Spin your front wheel. Anything on the screen? Adjust your wheel magnet. If you can't get any response from the head unit, take the magnet off and wave it past the sensor. If you still get nothing, try another magnet. If you still get nothing, then something is probably wrong with the computer and/or sensor.
If it's a wireless computer, replace the battery in the sensor.
If it's a wireless computer, replace the battery in the sensor.