First, open the unit up and look at the back circuit board. I'm sure there is other guides around for that or you can figure it out on your own.
The problem you need to solve is totally remove power from the board. Note that just turning it off isn't enough, as the chips etc will still have power. You can either (a) remove the battery for a while or (b) short out the system to force it to power down.
(Method A)
If you remove the battery, you may have to remove it for a while! On my friends unit it was still in a low-power mode, so even with the battery removed other elements were holding power for a while. I'd leave it off up to an hour to be 100% sure. If you want after removing the battery see method B. It's slightly safer to unplug the battery before doing Method B as you can't risk shorting out the battery entirely.
NOTE: Some people have reported that just unplugging & replugging the battery immediately solves the problem, without having to wait or short out as I had to do. Or with the battery unplugged try holding the power button for a while, that might also work.
(Method B)
To short out the power system, find one of the larger components on the circuit board, a tantalum capacitor (there may be several). You may need to remove the GPS antenna board. The capacitor will be around 3mm x 2mm to 6mm x 4mm and look like this:
The numbers will be different, but should have two terminals and a white stripe. To short it out short both terminals together with a paperclip bent etc. You may even see some very small sparks when doing this! Just do it momentarily, don't hold it on.
Finishing
Reconnect the battery if unplugged, and connect the GPS to a power source. The unit could get back in peril if the battery dies while it tries to boot, so it's better if it's plugged in! Plug it in and hold the power button for a while, and hope for the best.
manicman, August 2010
