Tools - options- display - layout elements-display layout and model tabs.
Not sure why they would dissapear but check the above.
I guess the other thing maybe that they have dissapeared behind the command line for some reason (not sure why though). Try dragging the command line down to see if they are behind or not.
Thanks for reply--it is actually not the tabs--sorry for mis-phrasing--but the border title block, the viewports, any paper space text. The layout sheet is empty...