macbook reboot loop, won't start in safe mode

My macbook was fine, I thought I'd finally do the os update, so I left it to it. Restarted and now I'm stuck in a reboot loop, I've been through tonnes of guides, safe boot doesn't work, neither does resetting all the other system stuff (holding option p and r etc tried about 15 guides before posting here) and to top it off, I can't get help from apple because apparently my serial number is wrong. But they set all of that up for me when I bought it.

I should add, I'm aware it could be a physical error. But I don't understand why I was fine watching a movie on there etc prior to the update and the macbook literally didn't move, not like I dropped it or something