Have you try to do DISK CHECK from windows? It seems to be an error in file system ( at my opinion). Try connect you N900 at mass storage, go to My Computer, right click to N900 storage and click properties, now go to tools and disk check, tick the box automatically fix file system error and click start, and wait.
Hope that fix the problem, if you haven't mess all over. Also some recovered files kept in seperate folder in root(hidden file so enable show hidden files on your PC) with name LOSTFILES.OO

Your last hope if it doesnt work is to use file recovery software (like recoverMyFiles) and try to find deleted files