Excel files now not opening.

Hi Leigh Foster,

Because the file is password‑protected and multiple older copies now fail on different PCs at the same time, I wouldn’t assume simple file corruption alone. This type of behavior can also occur when desktop Excel, a recent Office update, trust settings, or security software begins handling a protected workbook differently. The fact that Excel for the web can still open the file and display the data suggests the workbook contents may still be intact, even though desktop Excel is currently unable to fully open or re‑save it.

To narrow this down and attempt recovery, please try the following using a copy of the file only, in this order:

1. Copy the file from the portable drive to a local folder (for example, Documents) and test opening it from there. External or removable drives can sometimes trigger different security handling.
2. Open Excel in Safe Mode: press Windows + R, type excel /safe, and press Enter. Then try opening the workbook.
   • If it opens in Safe Mode, that usually points to an add‑in, startup item, or trust/security setting rather than the file itself.
3. Temporarily test with antivirus or endpoint security disabled or excluded, if possible. Password‑protected files can sometimes be scanned or blocked differently after updates.
4. In Excel, go to File > Open > Browse, select the file, click the arrow next to Open, and choose Open and Repair.
5. Since it opens in Excel for the web, try using Download a copy or Save As there first, then test the newly downloaded file in desktop Excel.
6. If you’re able to recover or rebuild the workbook, try saving the recovered copy as an Excel Binary Workbook (.xlsb), which can sometimes be more tolerant with large or complex files.
7. If exporting still fails, copy only the most important sheets into a brand‑new workbook as a recovery step.
8. If the issue persists across files, run a Quick Repair for Microsoft 365.

If possible, please also share the exact last line of the error message (in quotes) that appears when desktop Excel fails to open or export the file. That final line is often the most useful clue in determining what Excel is blocking on.

Hopefully one of the steps above helps you recover it, but feel free to update the thread with what you see and we can take it from there.

Please let me know if this proves useful to you, or if you would like further assistance.

I'm looking forward to your reply.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

The behavior described is consistent with workbook corruption affecting the main file and its recent copies, combined with Excel’s protection mechanisms when a file’s internal structure no longer matches the expected format.

Key points from the scenario and related behavior in Excel:

1. “Unreadable” or won’t open across multiple devices
   When a workbook’s contents no longer resemble the expected file format, Excel may refuse to open it on any device. This is described as a file whose “file name extension might occur when a workbook that you are trying to open is unreadable and the contents no longer resemble the expected file format.” In such cases, Excel treats the file as corrupted.
2. Why older versions can also fail
   If the same workbook has been repeatedly copied forward (month to month) and edited over a long period, any underlying structural issue can be carried forward into each new version. Over time, this can result in:
   • Internal corruption that only becomes severe enough to block opening after a certain change or save.
   • Multiple recent versions sharing the same corrupted structure, so when Excel’s validation becomes stricter (for example after an update), several past versions may all be rejected as unreadable.
3. Excel’s reaction to mismatched or corrupted format
   When Excel detects that “a file is in a different format than its extension indicates,” it treats the workbook as potentially corrupted. The guidance in that case is to attempt repair or recovery, or to have the file resent/saved in a different format if it came from another source. In this scenario, the equivalent is that the workbook’s internal structure is no longer valid for an .xlsx file, so Excel blocks normal opening.
4. Why the web version can show data but export fails
   The web version of Excel can sometimes render and display parts of a corrupted workbook that the desktop client refuses to fully open. However, when trying to export or save as a new file, the corruption in the underlying structure can still cause an error, preventing creation of a clean new workbook.
5. What can be done (within documented options)
   The supported options focus on corruption recovery rather than explaining a single root cause:
   • Use Excel’s repair and recovery procedures for corrupted workbooks. The documentation describes how to repair a corrupted workbook and recover data when you can’t open the workbook in Excel, including:
     • Setting calculation to Manual in a new blank workbook, then opening the corrupted file so Excel does not recalculate on open. This sometimes allows the file to open far enough to save a new, healthy copy.
     • Using external references from a new workbook to pull data out of the corrupted file (data only, not formulas or formatting).
   • If the workbook is severely corrupted, only partial data may be recoverable.

In summary, the most likely explanation for all recent versions suddenly becoming unreadable is accumulated workbook corruption in a complex, frequently copied file, combined with Excel’s format validation rejecting that corrupted structure. The recommended path is to follow the documented corrupted workbook recovery steps to salvage as much data as possible into a new, clean workbook.

References:

• A file is in a different format than its extension indicates in Excel
• Repairing a corrupted workbook
• Repair a corrupted workbook