"Start" is ambiguous. Let's dig into what you mean by that.

1) Remove any power strips, UPS devices, extension chords, or other items other than a single power cord between your wall power socket and Montage's power connector.
2) Turn Montage's power switch to the OFF position
3) Unplug Montage from the wall by removing the AC cord from either the wall socket or the back of your Montage and plug the AC power cord back in (so both ends are plugged in).
4) Turn Montage's power switch to the ON position
Question: Does anything at all happen? Any LEDs turn on? Does the touchscreen turn on and change from black to some other color? Does the screen display anything? Any of these would indicate a "start" of something. What is expected to happen - and what is failing to happen?

Thanks for replying,
Sorry by my english, I'll try to be more clear and specific.
The sequence was a follow:
1) The Montage was working properly. We decided to disable the Autopower off feature, using the manual procedure (turning on the Montage while pressing the first key). This operation was successfull, the Montage booted correctly and working fine during the sound check.
2) After the sound check, we turn off the Montage and unplug the power supply.
3) Before the live show start, my assistant plug the power supply and try to turn on the Montage but once it was pressed the power switch goes automatically to the OFF position. The touch screen blinks for an instant (grey color with some lines) . The leds did not turn on in any moment. We tried 4 o 5 times with no success, using 2 different power cords.
4) We sent the Montage to our a technical assistant in order to check the power input protection circuit and he found that everything was ok and that the problem might be at software level.

I live in Uruguay, and acquired this Montage 7 in Mexico city 3 weeks ago, so the warranty still valid, but Yamaha doesn't have any official representation in Uruguay. I really don't know what to do.

Thanks in advance.

You seem to be indicating that the physical power switch is rocking back to the "off" position even when you do not touch it. This isn't expected - the switch should only rock "on" or "off" due to the user pressing the switch and no other reason.

Service is supposed to start at the dealer or retailer.

Dealer information for your region is here: https://asia-latinamerica-mea.yamaha.com/en/find/5_latin_america/0_latin_american_countries/index.html

The repair form is here:

https://inquiry.yamaha.com/country/?act=2&lcl=en_WW&auth=none

or "service station"

https://inquiry.yamaha.com/country/?act=4&lcl=en_WW&auth=none

Hello Diego - I'd suggest re-enabling the 'Aurtopower Off' feature. It doesn't work half the time anyway, and if you set it to maximum (30 mins?) you only have to press one key occasionally to re-set it. Are you saying the switch physically moves? I often think it reverses whatever mode Montage is in, regardless of being down left or down right!

Hello Diego - sorry! Stupid suggestion if you can't start it up! Have you tried switching the rocker the other way before applying power? Or physically holding the rocker down while switching on? As it was working OK I assume the voltage settings match?

A unit that is powering itself off is likely doing so to protect itself. Use only the gauge power cable as came with your instrument.
What is the voltage situation where you are?

Same boot problem here, any solution? here is the video:
https://www.youtube.com/watch?v=x-SMdWx04mo

https://www.yamaha.com/paragon/servicerlocator/

or

https://uk.yamaha.com/en/support/service_locator/

or

https://nl.yamaha.com/nl/support/service_locator/index.html

or

...

(service in your area)