New user - still finding my feet with this powerful product.

My project has 8 sheets and all were set up with either A2 or A1 print configurations. Outside the viewport I had scale 1:1 and inside I had 50:1

Suddenly I find that one sheet no longer shows just a page it shows the model view and the sheet appears to have no boundary.  I can see the drawing I want to be in the sheet view but am struggling to get the viewport back in place.

All my sheets had a viewport configured for the page size.  On this sheet I cannot find the viewport. I have tried adding a new viewport by view, view tiles, single tile and then drawing a rectangle but to no avail - no viewport is created.

See also strange artefacts in background in second screenshot

What might I have done?

What can I do to find or reinstate the viewport?

Many thanks,

James