Anyone having trouble with Windows 11 today - specifically Excel?

[usonian]

3. On the theory that You (or I) can't be the first person to run into any one problem ...

I did a quick web search, and it could be that anti-virus is playing games.

https://spreadsheetplanet.com/remove-read-only-from-excel/
https://support.microsoft.com/en-us/office/why-did-my-file-open-read-only-3ab4b792-da50-4b38-8628-14c64e1f1d15

Doubtful that you messed up a setting (accidentally) to save files as read-only.
https://www.exceldemy.com/excel-this-workbook-was-opened-in-read-only-mode/

But people accidentally change settings. I do, and make a good (unpaid) living here at DU helping people figure out that they did, how, and how to undo. (iphones make private mode too easy to set).

HTH