Sounds like a corrupted Ai file…not that uncommon….
That has happen to me on several occasions (turned out to be corrupted fonts).

What saved the day was instead of “opening” the file (which I couldn’t), I created a new blank document (File>New Document) the size of the original then “placed” my original file (File>Place) into that.

That should at least get your file into Illi, so you can then fix whatever became corrupted.

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SWAMP