How to unlock workbook?

[ADT57]

When I select tools them protection all is greyed out but protect shared
workbook. I do not remember protecting any part of this work book. I want to
add a worksheet to this book. Under insert the worksheet is also grayed out.
Any help greatly appreciated.

 

[Luke M]

Shared workbooks have many features disabled. You'll need to first unshare
the workbook before protecting/unprotecting a sheet. It sounds like the
workbook was protected (preventing you from inserting a worksheet) and then
the workbook was shared (preventing you from unprotecting the workbook).