Heya! I've had this problem too after working on design layers and then the viewports within the sheet layers disappearing despite correct classes and layers being visible. What had happened in my case was that I had made viewports of say a piece of scenery made of two components which worked fine but then had disappeared after working more on the design layer - I realised that it was because I had 'grouped' the two components and therefore the viewport night longer recognised them in the classes they were in as individual components. Ungrouping made them re-appear straight away in the sheet layer - hope this helps!
