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? Usual suspects: trash the prefs, reinstall, delete and reinstall, close all other apps... Etc.
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)