Unable to set the Visable property of the Worksheet class

The title of this post is the Error message rec'd when trying to fix a problem i've never encountered before. I created a spreadsheet with a Hidden (not VeryHidden) tab. When the sheet went out to someone else to fill in and was returned, the tab could not be unhidden via the right click method. The only menu items available for selection were 'ViewAll', 'Protect Sheet' and 'Select All Sheets'. When i went to VBA to check the properties, Visible was indicated as the selection. When i fussed around to change it, the Error message (displayed in the Title) appeared. This is the case for all the tabs in the Workbook.

This workbook has been used in this fashion for over a decade. The same user returned 2 of these sheets and 12 others were returned, none exhibiting this problem.

I'm stumbling around trying to find a toggle that got flipped. I'm assuming it is going to be something simple and obvious but i'm not finding it on normal menu items. Yesterday was the first time i'd used the VBA feature. Looking forward to your feedback.
Cheers

It sounds like the Workbook Protection feature was turned on.

Thanks JP as i mentioned i tried the obvious. Is there a workbook protection toggle somewhere besides the 'Review' menu? I don't see anything on the VBA properties window.

Thanks JP and AlphaFrog. I mentioned i had tried the obvious. However, in actuality i gave the ProtectWorkbook menu item a pass because it was titled 'ProtectWorkbook', unlike the 'ProtectSheet' menu item the Workbook item does not change to UnProtect when Restrict Editing has been enabled. The workbook came to me protected and i'd used the password to gain access and that usually means we are good to go. Access granted, but still restricted at this level. Lesson learned, thank you very much.