How to return back to the default view of pages in Pages?
Welcome to Ask Different! 

1) Untick the "Facing Pages" option on the right sidebar in the "Document" tab (the open tab in the picture you have shared). This should remove the diagonals.

2) Click on the "Zoom" pull-down menu on the left on the toolbar at the top and select "One Page". This should bring a next page below the previous one. 

I highlighted these in red boxes in the screenshot below.

Hope these help.