Damaged File - No reason, can anyone help?

Here is a link to Adobe's tips on how to recover a corrupt document.

https://helpx.adobe.com/indesign/kb/troubleshoot-file-issues.html

I skimmed the earlier link and didn't see the Adobe email in it—doesn't mean it wasn't there—so here it is again just to be safe:

If no resolution works, share the affected file with us at idfile@adobe.com. We'll investigate the issue and let you know if the recovery is possible. When you share your file(s) with us, you not only enable the Adobe team to find a solution to your problem, but also help us improve the overall user experience by identifying and resolving other similar issues. We assure that your file will strictly remain confidential and will be used only for investigation purposes. Ensure that the file you share does not include any sensitive personal information. We'll try our best to assist you, but as recovery depends on various factors, we cannot promise a permanent solution.

There is also a paid, third party solution: https://markzware.com/products/file-recovery-service/

~Barb

Try the blind open idml found in this thread. 

If that doesn't work, there is also an Adobe address in that thread that you can send a corrupt file to; they may be able to recover it.

You can try opening a copy but beyond that I have no idea. Adobe had a service where they'd try to repair it but I don't recall if they still do or how to send them.

If you were working in a Dropbox or OneDrive folder you could roll back a version and try that.