my 54 page document suddenly turned into 54 single pages on the computer screen, spread nine pages wide and six pages down.
How do I restore it to single successive pages on the screen?
What is the scale value you see in the status bar, in the lower right corner? 20%? And if you try to increase it to 100%?
Sorry, I did not see this before answering.
On the bottom right hand margin there is a little slider control that controls how big the page appears.
If it is fully left, you will have multiple pages on the screen, if it is fully right, you will only get part of a page on the screen.
Move that slider control to suit your taste.