WPT Memory is Full - see Waypoint Memory Full. Occassionally these units will simply fail to find any satellites when first turned on. The solution is to turn off the unit and turn it back on. The second time it always seems to work. If you have moved more thatn 300 miles or had the unit off for several months you will need to reinitialize it before you can use it.
Unit fails to turn on All Units Check the batteries.
Be sure they are install correctly. It is also possible you have water damage. Dry the unit out and try again. If this fails you may need to return the unit to Garmin for repair. If desperate you may want to read the chapter on secret commands and see if you can revive the unit yourself but you are likely to lose all user data.