First, try resetting the unit and seeing if that does the trick. To do that:
To perform the master reset:
- Power device off
- Press and hold Lap/Reset and Start/Stop
- Power device on while still holding both buttons
- Continue holding buttons when the Garmin "splash" screen appears
- Release buttons when Garmin "splash" screen disappears
The Edge 800 is now reset. Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data.
It may not solve your problem, but is the recommended initial solution.