Argh, half way to Maine, my beloved Zumo 550 locked up dead and wouldn't turn on!
I fixed it, but not without killing a few precious riding hours and lots of curses. Everyone taking long rides with the Zumo MUST ALWAYS TAKE A 3/64 ALLEN WRENCH and USB charger along on trips... here's what I did:
1) Remove battery from unit using 3/64 allen wrench (took a while to find)
2) With no battery in place, plug in USB type charge adapter (used the one from my cell phone through bike's accessory port) into Zumo unit, and allow it to boot up into mass storage device mode.
3) Shut down, pull plug, replace battery. Should turn on now.
Everyone should be advised to update to latest firmware, read all the bugs on http://garminzumo.wikispaces.com/BUGS
and sift through http://www.zumoforums.com
before they really start to depend on these units for long trips. I still love the Zumo 550, but I learned my lesson, and I also make sure always to bring maps as backup.