Excel won’t save. My Authenticator hasn’t requested a code for probably close to a month

For the Excel save issue, start with the fastest isolation steps:

1. In Excel, use File > Save As and save the workbook with a new file name.
2. If that fails, try saving it to another location.
3. Create a new blank workbook and try saving it to the original location. If the new file saves but the original one does not, the problem may be with that workbook.
4. If possible, save the file as a different Excel file type such as .xlsx or .xlsm instead of .xls.

Common causes supported by the documented troubleshooting are:

• Missing folder permissions: Excel needs Read, Write, Modify, and Delete permission to save.
• Not enough free space at the destination.
• Antivirus software interrupting the save process.
• A third-party Excel add-in interfering with saving.
• File path too long: if the full path and file name exceed 218 characters, Excel may show Filename is not valid.
• Another user or another Excel instance locking the file during save.

If add-ins may be involved, test in Excel Safe Mode:

1. Exit Excel.
2. Select Start, point to Programs.
3. As Excel starts, press and hold Ctrl until the safe mode prompt appears.
4. Select Yes.
5. Open a new workbook and try saving it, then test the problem workbook.

If saving works in Safe Mode, a third-party add-in or startup file is the likely cause.

For AutoSave specifically, it is enabled only when the file is saved to OneDrive, OneDrive for Business, or SharePoint Online. AutoSave is disabled if the file is:

• saved locally or to another cloud service
• on SharePoint on-premises
• in an older format such as .xls, .ppt, or .doc
• in a local OneDrive folder while OneDrive sync is paused
• being synced by an older OneDrive sync app
• embedded inside another Office file

If the workbook is in Excel and AutoSave is off, also check whether it uses features that block AutoSave, including:

• Shared Workbook
• password encryption

For password-encrypted workbooks, the documented approach is to remove the password from File > Info > Protect Workbook, then apply a sensitivity label from Home > Sensitivity if needed.

About the missing Authenticator prompts: the available material only supports that if a workbook is stored on OneDrive, SharePoint, or another cloud location, Excel may prompt for Microsoft 365 sign-in rather than a workbook password, and signing back into the Microsoft account in Office can resolve that type of prompt. It also notes that if every workbook prompts for credentials, the issue is more likely related to Microsoft 365 sign-in, Windows credentials, or organizational security policies.

A practical next step is to sign out of the Office app and sign back in, then test saving again.

References:

• Troubleshoot why Excel doesn't save changes
• What is AutoSave?
• Excel password - Microsoft Q&A