935 blocked after only 3 months

a. Hold POWER BUTTON on for 15 seconds. The device should hopefully start to turn off.
b. Press and hold UP ARROW BUTTON and DOWN ARROW BUTTON
c. Press but don’t hold the POWER BUTTON for a second or so – device should turn on
d. Wait for 1st beep then release UP ARROW BUTTON
e. Wait for 2nd beep then release DOWN ARROW BUTTON

Try to do a hard reset with the button of your watch
It is something like:
- Power off your watch (Hold "light" button between 15-20 sec)
- While holding "start" & "back" button, hold few sec "light" (to power on the device)
- Release "start" button after the first beep and the back button with the next beep

(for me it was not working with the "up" & "down" arrow button, but ok with the "start-stop" button)

Thank you very much for your help, this evening when I can pick up my watch again, I will try to do this reset and then I will let you know.

Try to do a hard reset with the button of your watch
It is something like:
- Power off your watch (Hold "light" button between 15-20 sec)
- While holding "start" & "back" button, hold few sec "light" (to power on the device)
- Release "start" button after the first beep and the back button with the next beep

(for me it was not working with the "up" & "down" arrow button, but ok with the "start-stop" button)

YES THIS IS THE SOLUTION!
I don't understand why Garmin doesn't write in the manual this possibility!!!!

More important now. Why did it crash? What were you doing? What have you added to the device? What sensors were connected.

More important now. Why did it crash? What were you doing? What have you added to the device? What sensors were connected.

Why did it crash? I do not know
What were you doing? a very normal race
What did you add to the device? only 2 data fields. Dosen Run and Race Screen (used for months without problems)
Which sensors have been connected? none, only the smartphone connected to the live track and the bluetooth headphones with spotify.

I was running with Dozen Run visible on the screen, active live track and music in my ears with bluetooth headphones and spotify.

After a few miles I realized that the screen was blocked (only the screen, the clock continued to work because every km in the headphones the voice told me the speed and after a test from home, the live track had worked perfectly)

I tried to press the keys to change the screen or start and stop, but nothing worked and the screen remained locked.

So I thought to do a soft reset by pressing the "light" button for 15 seconds, it turned off, but when it came back on it stopped at the image posted in my first post without any possibility,
(the computer and smartphone obviously did not recognize it)

I feel honest because I bought from Amazon, they had already accepted the return with full refund, but thanks to the solution read here, I deleted the return. Now I just have to reset everything in my 935

Mine had a similar crash last year. In my case I had/have nothing extra added. I was following a course with alerts set for 20 mins and 5km laps. Somewhere along the line it went belly up, I assume because the processor got overloaded. It hasn't happened since, but then I haven't used it with similar settings since.

I forgot maybe an important thing. I left home with wifi (important for the smartphone for live track and spotify), after leaving home, dropped the wifi connection, the subscription to the 3g data plan of the phone was terminated expired so no data connection for the smartphone, and spotify stopped, but everything kept working perfectly and I found the screen stuck after over 11 km and only at 15km I stopped the session to try a reboot.

BRILLIANT!  Thanks gilles.k for your help!

Garmin, why did I have to search for 30 minutes to find this?