See solution in other versions of Excel:
Question: In Microsoft Excel 2007, how do I unhide a sheet that has been hidden?
Answer: Right-click on the name of any sheet and select Unhide from the popup menu.
When the Unhide window appears, select the sheet that you wish to unhide. In this example, we've selected Sheet1. Click on the OK button.
Now when you return to your spreadsheet, Sheet1 should be visible.