If it happens to only one document or some documents it may be corrupt.
One thing often tried first is to create a new document and File>Place the (PDF contents, if any, of the) corrupted one to see how much may be rescued that way.
Here are some websites where you can see whether it can rescue the actual file, and if it can, you may pay for a subscription to have it done
Do you see this as soon as you open illustrator? Does it crash the app? What version of ai? PC/Mac? What have you tried already for repair Illustrator file? Usual suspects: trash the prefs, reinstall, delete and reinstall, close all other apps... Etc.
Save early. Save often.
Also you can upload the file here https://onlinefile.repair/en/illustrator.html and than reopen it.
Adobe is pretty obstinate (is that the correct word) when it comes to updating their software. Have you contacted them to find out why it won't open?