Can you upload a copy of one of the workbooks in which you're experiencing this?
As for 'Unhide...' not being available, are you sure there are hidden sheets within the workbook? (If they were hidden using 'xlSheetVeryHidden' in VBA, then you won't be able to unhide them using the context menu of the worksheet tab.)
Just for giggles, you could run this macro in the VB Editor window:
The Ply commandbar object is the context menu (right-click menu) for worksheet tabs. This will reset it to default. (Again, if there are no hidden sheets the 'Unhide..' option will still be grey.)
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