If it were my unit, I'd probably think I had a corrupted Waypoint or a bad GPS module:
I would do this:
With the unit off, unplug the gps from the back of the unit.
Check the gps cable for kinks/frays.
Check the connecting pins for the gps at the cable and back of the unit.
Turn the unit on.
If possible, Export All Nav data to an SD card.
Bring the SD card file into Humminbird PC and go through and verify
that you don't have any duplicate Waypoints or bad looking coordinates.
When that is done, if possible, Erase all NAV data from the unit.
Re-load your Waypoints onto the unit from the SD card.
Shut off the machine.
Then, hook the gps back up, turn the machine on, and look at the gps diagnostic and
see if the unit recognizes the gps.
If all else fails, set unit back to default settings and try again.
Good Luck.