This message means that something happened since the last auto-save which caused the document to become corrupt. Your best bet is to click "Yes" when this message appears, since this will recover the most recent auto-save. Then you should immediately save once the auto-saved file opens. This option to recover the most recent auto-save is new to LayOut 2014. If you use the default auto-save interval of 5 minutes, then you will have lost at most 5 minutes of work.