Ok, this has happened to me, occasionally.
Follow these steps:
1) Connect your 505 to your computer and copy everything to your HD. This is only for protection.
2) Disconnect.
3) Open the Reader Library program and turn Sync on, if you do not normally use it.
4) NOW: Your Reader Library, I am assuming, shows all the books that would normally be on your 505, with your categories, etc. (Remember, you went to a lot of trouble creating YOUR categories and putting books into those categories.)
5.) Good. Now connect your 505 to your computer. Everything will be re-synced back to your 505.
NOTE: The above is for lrf/lrx files. I don't use epub because I think it all too often looks horrible - really horrible - so I have NO IDEA if the method above works for that format. Your opinion - or the opinion of someone else reading this method - may differ. That's "jim-dandy," as we say in the South.
Don