That happened to me a couple of months ago on my K3. Mine is now over 2 years old and I looked away for a second and when I picked it back up, I had this same full black page.
I thought for sure it died.
But paging forward in the book made it normal again. I tried turning it off first too and it didn't go away that way. But turning the page did.
Then it happened again a few days later. So far I haven't seen it again. But I also have been using my PW more often.