I don't know how to describe what exactly was going on with the plansheet but here goes nothing...
While edited a plansheet, I noticed that when I deleted a viewport, the model space was still attached to the paperspace. After deleting all the viewports, the model space was still present and was not deleting, despite all efforts to get rid of it. It was almost as if the entire plansheet had become a viewport itself and would not allow me to delete it.
Does anyone know what is happening? I was able to resolve the issue by deleting the sheet and copying over another sheet, but I would like to be able to solve this issue in the future.
Layouts can become corrupt. It's unexplainable as to what causes it. Crashing while that layout is current is sometimes involved, but not always. Using templates from years past can propagate corruption. Fresh templates filter out corruption.
Next time, if you can, get screenshots and videos. That would help your friends here recognize an issue.
Better yet, post the drawing.
Is it possible you accidentally double clicked on the viewport which would make it full screen?