This hard reset instructions worked for me:
Hold the light button until the device turns off.
Hold the two right buttons, <Start/Stop> and <Back>, while turning the device on by pressing and releasing the…
Thanks a lot, my 935 freeze completely in the middle of a run today.
I can force shut down but not able to restart it (stay on the blue triangle forever).
This procedure works to reset and bring it back…
Try hooking it up to your computer and see if you're able to sync with Garmin Express. This worked for me with my FR235 in the past. I had the same thing happen with my VA3 a couple of months back, and unfortunately, nothing helped and I had to exchange it. If this is the case, you're best off calling Garmin Support Monday then emailing. Best of luck!
Update: If I recall correctly, someone said they tried the hard reset while connected to their computer. Perhaps this will work?
Too add to the above, if that fails you might need to do a full reset back to factory defaults. That's the first thing Garmin will have you do.
@cyril_E
Look from Page 28 in the user manual
If none of that works try the procedure below. Note that you will lose all data. The watch will be as when you bought it. If you search through the forum you'll find posts on how to make this less painless by backing up certain data and restoring it afterwards.
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This hard reset instructions worked for me:
Hold the light button until the device turns off.
Hold the two right buttons, <Start/Stop> and <Back>, while turning the device on by pressing and releasing the light button.
Wait for an audible beep and release the <Start/Stop> button.
Wait for a second audible beep and release the <Back> button.
You will lose all your user settings and activity history. There are steps you can take to recover user settings etc, but performance stats will need to be regenerated over time.