So the theory goes that Bran got stuck in the body of the first White Walker, aka the Night King.
One theory suggests he also went back to the time we saw on screen when the Children of the Forest created the Night King out of a human man.
It doesn’t explain everything, and there are huge motivation holes (is Bran’s Night King marching south because he knows this is what’s supposed to happen, like Doctor Strange knowing the “endgame”?), but it’s a theory.
Bran had been warned that you can’t remain in the past for too long, or in someone else’s body for too long, because you can get stuck.
…
Emilia Clarke's Unsettling Reaction To Daenerys' Final Game of Thrones Scene