Dear capmagn,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, please don’t worry, we can work together to narrow down and resolve the situation.
On my side, when I click on Developer tab the Macros buttons is active in Excel 365 app. See my test screenshot for your reference:

This can be occurred if you have a macro running.
If you press Alt + F11, will the VBA Editor have shown up? If you can see the VBA Editor, please check whether there is any macro running.
Also, VBA editor (Alt and F11) and click on Run button and click on "Reset" (it is a square blue button on the tool bar) and click it.
If the problem happens to all Excel files including a new created one, the problem is more related to the Office app itself or your computer environment. You can first try suggestions below to see the result:
- Make sure you’ve updated Office to the latest version that some issues can be resolved with such method.
- Try open your Excel insafe mode which allow you to safely use it when it has encountered certain problems
- If still can’t work, please try torepair an Office application to fix the problem.
In the meantime, I can’t reproduce the issue and to confirm whether this is a bug in a specific version, please provide us the version information. To check version: Open Excel>File>Account>Product Information and capture a full screenshot and share here and we will be tested on that version and verify the result with you.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad
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