Hello Stasia,
If you follow the Adobe Animate forum for a while you will find out that FLA file corruption is a very common issue is recent years.
There is a new thread, posted by some desperate person asking for help, almost every day.
Here are a couple of recent examples:
https://community.adobe.com/t5/animate-discussions/erreur-d-ouverture-d-un-fichier-adobe-animate/td-p/13096604
https://community.adobe.com/t5/animate-discussions/question-an-error-occurred-opening-file-adobe-animate/td-p/9405336
https://community.adobe.com/t5/animate-discussions/an-error-occured-while-opening-the-file/td-p/13121212
https://community.adobe.com/t5/animate-discussions/empty-symbols-problem-corrputed-flash-file/td-p/13073865
The list is endless.
These pepole are not EDAP Tools users, and their posts are never addressed by members of the Adobe Animate dev team.
The only thing you can do to prevent data loss is to save versions (the way you do) frequently, make daily backups and avoid using features prone to data corruption, such as Layer Parenting.
Personally, I still use Flash CS6, because I find Animate unreliable and underperforming.
If backup files that have opened fine in the past suddenly cannot open now, you can try to open them in a different version of Animate (you can have multuple installed at the same time) or on a different computer to rule out issues with your installation. If the files are really corrupted, they will not open anywhere.