They could be just barely off the screen:
Window|Arrange|tiled
and resize the way you like.
You could have hidden the tabs:
Tools|options|View tab|check Sheet Tabs
You could have dragged the horizontal scroll bar over the tabs.
You can try to grab the left-edge of the scrollbar and drag to the right.
Or you could use a line of code (which may be easier).
Open excel
Open your workbook (with the problem)
hit alt-f11 to get to the VBE (where macros live)
hit ctrl-g to see the immediate window
type this
ActiveWindow.TabRatio = 0.5
and hit enter