I second Angelo's encouragement to keep trying. I too have been in this predicament more than once and ready to throw a nuvi at the wall. As Angelo said, the timing is imperative, you must press immediately the logo appears.

Here are some other tips:

Make sure you have updater.exe open on your desktop with the .rgn file ready to flash, and the dot entered beside 'USB device'. When loader appears, press OK immediately.

Plug the usb into a REAR port of your PC, have your thumb hovering over the screen ready to pounce, plug the microUSB into the nuvi and the instant the logo appears immediately press down hard with your thumb covering the G and a little above it as well using the whole of the pad of your thumb, not just the tip. If 'Loader' does not immediately appear in the top part of the screen, you've missed it. Hold the power switch down for at least 10 seconds before you try again.

Just keep trying. We've all been thru this. If your nuvi powers on as far as the logo it can go into pre-boot. If it can go to pre-boot it can be recovered. I once wasted half a day to get into preboot to cure a badly bricked 3790. That was one of most frustrating days of my life but after dozens, probably over a hundred attempts, 'Loader' appeared and it flashed the cure fw. Stay calm and be organised. There is nothing worse than finally getting into preboot to find you're not ready with updater.exe and the rgn file.