In the examples below, one viewport has been placed on a sheet.
Example 1: The viewport has been selected and Hidden in View > By Element.
Solution 1: Unhide the viewport.
Solution 2: Enable the Select Pinned Elements selection option, and unpin the viewport
Example 3: The viewport has been activated
Solution 3: Deactivate the viewport
Example 4: All Annotation Categories have been turned off in the sheet's Visibility Graphics
Solution 4: Make all Annotation Categories visibile
Example 5: The viewport is part of a Selection Set Filter and the filter is turned off in the Visibility Graphics
Solution 5: Turn the Selection Set filter on