All excel file suddenly become read only

I’m sorry to hear that you’re having trouble with your Excel files. Here are some potential solutions that might help:

Check the File Properties: Right-click on the Excel file and select ‘Properties’.
If the ‘Read-only’ attribute is checked, uncheck it and click 'OK’.

Antivirus Software: Some antivirus programs may protect you from potentially unsafe files by opening them read-only.
You may need to adjust these settings.

Protected View: Files from the Internet and other potentially unsafe locations can contain viruses, worms, or other kinds of malware that can harm your computer.
To help protect your computer, these files are opened in Protected View. You can change these settings.

OneDrive Storage: If the file is stored on OneDrive and your OneDrive storage space is full, you will be unable to save the document until you are under your allotted space.

Product Activation: If Microsoft 365 or Office isn’t activated, or your Microsoft 365 subscription expired, you might get an Unlicensed Product message or an activation error.

If none of these solutions work, you might want to try restarting your computer, installing Office updates, performing an Online repair of Office, or checking your internet connection.