Place the damaged Illustrator file in a new Illustrator file - it is one of the methods that can help
Otherwise see helpful topics about .AI file restoration
When your .AI file is hardly damaged, you may refer to powerful solution https://illustrator.recoverytoolbox.com/ Recovery Toolbox for Illustrator
I've discovered a fix that worked for me in un-corrupting corrupt adobe Illustrator files - quite by accident. Hope this helps someone!!
Requires Acrobat (not just the Reader).
Works on (and was tested in) Illustrator version 10 and all CS versions.
>Open the corrupt file in Acrobat.
>Take the arrow tool.
>Click on any element or text in the document.
>Just move the object by hitting one of the directional arrows on your keyboard.
>Open the file in Illustrator.
>It will say 'this file has been modified outside of Illustrator' or something to that effect. it will ask if you want to keep changes.
>Say 'Yes' (or OK)