Had a prob with my newly acquired Zumo 550, it froze up and refused to power up. The manual only suggests pressing the power on and + buttons but that didn't cure the problem. Had to check through the Garmin support web site and found that you can remove the battery for 5 secs.....worked a treat!!
May help someone else who has the same problem.
From the web site:
Question: Why wont my zumo power on?
Answer:
To troubleshoot issues with the zumo powering on:
Connect the zumo to a power source such as the USB cable and your computer.
Hold the zoom (+) and ON buttons for 5 seconds. This restores power to your zumo.
If this does not restore power, you can remove the battery for five seconds then reinstall it to reset the zumo. Ensure that your zumo has the latest software updates.

May help someone else who has the same problem.
From the web site:
Question: Why wont my zumo power on?
Answer:
To troubleshoot issues with the zumo powering on:
Connect the zumo to a power source such as the USB cable and your computer.
Hold the zoom (+) and ON buttons for 5 seconds. This restores power to your zumo.
If this does not restore power, you can remove the battery for five seconds then reinstall it to reset the zumo. Ensure that your zumo has the latest software updates.