Brand new, first day vivoactive 5 screen blacked out, no reset, won’t turn on. Vibrates.

I'm having the same issue, and I do have the green lights flashing on the back 

This happened to me a couple of times. Sometimes it returns spontaneously. These are fixes I found in other posts: 

Find my device

Top button once, bottom button twice

To fix it, I synced the watch (it was attached to my phone still) and then I held down both buttons for about 10 seconds until I saw the green HRT lights go off on the back of the watch.  Note that while holding both buttons down, it will vibrate 3 times quickly after about 5 seconds -- you need to keep holding them down for at least 5 more seconds after those vibrations.  At this point the watch is off (you can be sure once you see the green light on the back go off).  Let it stay off for about 10 more seconds, then hit the buttons to power it back on (hold the top button for a couple seconds, I think). 

Pressed the top right button for about 15-20 seconds and the screen started to show again

Mine has the same symptoms. It appears to be a common flaw with the device. Plugging it into a computer seems to put it right (not useful when you are out and about and it happens).

I've reported it to Garmin support, here's what I've learned:

- needs to be Garmin support for your region, else they don't care

- they require a high fidelity photo of the front and back of the watch

- they require a video demonstrating the problem 

Encouragingly, they've asked for a shipping address.

Update - provided video evidence, they shipped me a replacement vivoactive 5

Within 24h, the replacement also demonstrated the black screen problem! Made a video of that too. Now Garmin are offering a refund. No admission of the flaw.

*Make sure you video the problem!*