This question invariably gets the spams from file repair.com sites or by proxy to another post where they spammed. If they are that good, there would be a demo version.
This issue is all about keeping our files safe on backups. There is another cause which is a patch or update to Illustrator that wasn't installed. This is on the https://forums.adobe.com/community/illustrator forum.
In fact, you should be posting this question there as that's where the well is.
Illustrator .ai file get corrupted. “The file is damaged and could not be repaired.” Yesterday I saw such message, when I was trying to open my .ai file.

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