Cause: The workbook window is sized in such a way that the tabs are hidden.
Solution:
This problem sometimes happens after you unhook the workbook window from the Excel window (by using the Restore Window button in the trio of buttons on the title bar), and then inadvertently move the window beneath the status bar. This may also occur if your computer's screen resolution is higher than that of the person who last saved the workbook.
To maximize the window so that you can see the tabs again, double-click the window's title bar.
If you still don't see the tabs, try this instead: On the View tab, in the Window group, click Arrange All. Under Arrange, click Tiled, and then click OK.
Cause: The Show sheet tabs setting is turned off.
Solution:
First, see if the option is, in fact, turned off. To do this, click the Microsoft Office button 
, and then click Excel Options. In the Advanced category, under Display options for this workbook, look at the Show sheet tabs check box. Is it cleared? If so, select it, and then click OK.
In some cases, you may not see worksheet tabs after adding a macro sheet to a workbook. For more information, see Worksheet tabs disappear when you change some "Display options for this workbook" options and then add a macro worksheet to a workbook in Excel 2007.
Cause: The horizonal scroll bar obscures the tabs.
Solution:
If you widen the horizontal scroll bar at the bottom of a worksheet, you may unintentionally hide some or all of the sheet tabs. If this happens, point to the button next to the left of the scroll bar:
When your pointer becomes a double-headed arrow, drag it to the right until you see the tabs again.
Cause: The worksheet itself is hidden.
Solution:
To unhide a worksheet, right-cick a visible tab on the worksheet, and then click Unhide. In the Unhide dialog box, click the sheet you want to unhide, and then click OK.
