If workbook protection has been enabled in the current document, you can disable it at any time.
To do so, choose the ribbon command Review | group Protection | Workbook protection 
again (you can recognize an active workbook protection by the fact that the icon appears darker).
As a result, users will be allowed to add, delete, rename and hide/unhide worksheets again.
Tip: The current state of workbook protection is saved within the document. And: If a password has been assigned, users will be required to enter the password to disable workbook protection.