OK
I think the problem may be with what I did later.
Firstly, I updated the nuvi to Firmware 4.00 via Webupdater.
As far as I remember, the Nuvi installed this update normally once I disconnected it from the PC.
Immediately after, I then updated the Mapping software to CN Europe 2012.20 via Mapsource & put its gmap3d image (now as gmap3d1 as I already had an existing 3d image from CN Europe 2011) into the Nuvi Root directory thru My Computer in accordance with instructions on this forum, also put in the JVC file.
I left it to transfer.
When I returned, and disconnected the Nuvi from the PC, the Nuvi remained in PC transfer mode, and its then that i couldnt switch it off.
The screen is totally unresponsive, and doesnt respond to the Power/Lock switch.
I connected it to an external charger to see if that would give it a boost or unlock it, but no change there.
I reconnected it to the PC, also that did nothing, and now, as the unit has locked up, it doesnt show at all in My computer.
Its in limbo.

Battery was fully charged, I didnt interupt the installation, and the unit did install the updates once removed from the PC.

Caty, I cannot get into any mode, as both screen and power button totally unresponsive.

Now not sure if it was the update or the transfer of gmap3d & JVC files that has it locked up.

I'm considering just letting the nuvi battery run down, cos I cant do anything else with it.

Really dont want to send it back to garmin because if they look inside, they will see a fair few maps that cannot be accounted for on my account with them, if you know what I mean.