If workbook protection has been enabled in the current document, you can remove it at any time. (You will recognize active workbook protection by the fact that the icon has a dark color.)
To do so, choose the ribbon command Review | group Protection | Workbook protection 
again.
This disables workbook protection again. You can then insert, delete, show/hide worksheets, etc., again.
Tip: If you save a document with workbook protection enabled, workbook protection will be automatically re-enabled the next time the document is opened. Furthermore, if the protection is secured with a password, it can only be disabled with the correct password.