For "Hidden" tabs - Go to the "Tools" menu. Select options. On the "View" tab look in the bottom right hand corner. There is a check box next to "Sheet Tabs". Make sure this bok is checked & the tabs should now be visible.
If you mean that the actual sheet is hidden the go to the "Format" menu, select "Sheet" and then select "unhide". You'll be given a list of sheets that are hidden. Unfortunatley you can only unhide them one sheet at a time.
HTH