Right click in the worksheet tabs section, select unhide , then select the name of the worksheet you want to unhide from the dropdown box. It will show all hidden worksheets in the workbook.
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If the tab is missing somehow, for excel 2007 & later, click on Home tab, then cells > format then you can see hide and unhide just click unhide sheet. Please link here for more help: https://support.office.com/en-us/article/Hide-or-show-worksheets-or-workbooks-69f2701a-21f5-4186-87d7-341a8cf53344