I had this problem with my 296 and Garmin suggested the following solution, which worked:
- Take the unit somewhere outside, in clear view of the sky.
- Press and hold the Out button, while you press the On button, and keep the Out button pressed in until you have to acknowledge the "I agree" message.
- When the power up sequence completes, the GPS page should display "Auto Locate", instead of "Acquiring Satellites". When it does, place the unit on the ground and leave it for up to 30 minutes, until "Auto Locate" is no longer displayed.
- After this, it should work normally.
Apparently this happens when the unit gets completely lost, so doesn't know which satellites are in its field of view.