Make sure you know where your class was stored in the first place.
I`m guessing that you use a setup like: Document.fla/Document.as (as Document class)
Per default the document class is stored in the same drectory as your fla but you should always separate your code from your document.
This can be easily done by using a folder structure.
For example if you create a folder in the folder wherre your fla sits that is called src, you would link the document class in the fla with src.Document (and the Document.as would sit in the src folder of your filesystem)
Now when your flash crashes and you want to reconnect with that document class and you click on the pencil in the Document-class icon, it will default back again and crfeate an empty class for you in the old folder.
So at first sight it would seem you lost your work (because its empty) while you only would have to manulall point flash to the right place (the src folder) where the class sits you used before the crash.
Hope that makes sense to you?
Other than that: I have never encountered the corruption of a class file (its only text, nothing there to be corrupted)