Power unit down, press pages key and power key, hold keys until USA map appears. This will not delete waypoints or other user saved data.
This method will delete waypoints and trails so be sure to back them up first unless you have no choice.
Shut unit off, press Zoom in, Zoom out, and the power key, hold keys until USA map appears. Unit will reset to factory default, you will need to allow time for unit to reinitialize gps position. This may take up to a few minutes on the first gps position lock.