[James Merritt]

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:
https://www.fixtoolbox.com/illustratorfix.html
https://helpx.adobe.com/illustrator/kb/troubleshoot-damaged-illustrator-files.html