"Workbook is Protected and Cannot Be Changed"

[DOUG]

This is news to me, since I created the workbook. The Security Center and
the Protect/Un-Protect features do not indicate password protection in
existence now. I am able to change and save the spreadsheets. I want to
re-title a spreadsheet tab. When I attempt to do so, however, I am
confronted by the error message in the subject line. Suggestions would be
most welcome here. (MS "Help" in Excel and online does not offer a clue, by
the way).

DOUG