My apple watch (series 6) comes on for a brief second but then goes off and won't come back on again

If I wait 5 minutes and then tap the screen and it comes on but again only for a brief second or two.

All the in app tools like heart rate monitor are working but I can't access the them on the watch only on my phone.

I've played with the always on button / wake on wrist etc buttons / wake duration buttons but I can't seem to fix it.

If I push the bezel a few times it starts talking to me about random stuff (I've tried the push 3 times fix, doesn't work) and will speak at odd times.

I've tried a lot of fixes - unpairing / running the battery down / hard reset / changing the face etc and none work

Recently when it does come on for a split second, there's a white box around the top right of the screen.

I'm exasperated! Can anyone give me some advice?