I think this post might be helpful for those of you who likes expereimenting with your Garmin devices, especially considering amount of "customised" software, applications and other products available through posts in this forum.
The problem I experienced and recovered from can hit any of you and I swear, I could not find direct solution to that problem, but accidentally bumped into it and now would like to share with everyone.

First of all, you need to recall what you have done to your Garmin device just before it stopped working/booting up, as failure to boot is most likely caused by a user not following firmware upgrade procedure or trying to "tweak" the device (as in my case).

I used customised Garmin Mapsource, which allows saving custom mapsets to HDD and other media on the PC, as opposed to official Mapsource that can only save on your Garmin device removable drive/card.

OK, I created custom mapset out of Topo Australia v3 (jusr few tiles) and saved to my C drive. Then via Windows Explorer I copied that gmapsupp.img file into Garmin folder of my Nuvi 255W. After that my device stopped booting up. When standalone, it was just reaching "AGREE" screen and then shut down after I press "AGREE" button. When I was connecting it to PC, the boot screen was going in cycles.

Searching web for solution, I found how to reset Garmin to factory defalut settings (press & hold lower right corner of touch screen whilst sliding power button) but it did not help.
Another thing I tried was to press and hold various areas of touch screen, while switching the power on (even WhereIs icon!!!). The only feature I discovered was: on standalone unit press & hold top left corner and switch power on brought me to some screen with crossed lines and dot with directions to press that dot. After pressing, dot was jumping to new position and I followed it, but with not much luck, device was shutting down after few attempts of that cat and mouse playgame.

The solution I came to by "try and fail" experience is:
1. Connect Nuvi to PC with USB cable.
2. Press and hold upper left corner of touch screen.
3. Slide and hold power button in ON position until:
4. PC will detect Garmin device hardware and Nuvi's screen will stop at initial logo screen.
5. Now device is accessible via PC browser again!
6. Browse via Explorer to Garmin device and remove the file that stuffed up booting of the device.
7. Disconnect device and reset power in usual way.
8. It works again!:D

=DISCLAIMER (or some sort of that sh*t)=
It worked on my Nuvi 255W and I saved $80 uncoditional repair cost + postage if I'd sent it to Garmin Service Centre. I guess it worth trying if your device is stuffed up. But it is up to you to decide whether to muck around like I did or come and drop on your knees in Garmin Service Centre (closest to me would be few thousands kilomenters, though).
OK, use this guidance at your own responsibility and don't blame me if you hands grow from the part of your body other than shoulders or you're in lack of lateral thinking.