Did you previously have "hide extensions for known filetypes" enabled? That way you can't accidentally alter the extensions when renaming (for example .doc, .mp3, etc.). When you can see the extensions, and rename the file, without taking care that the extension doesn't change, it will rename your file to a name without an extenstion. "Queen_Innuendo.mp3" could become "Innuendo" without the extension. Windows doesn't understand hat kind of file it is anymore, and therefore can't open it. The icon of the file will alter to a standard windows flag, instead of the icon it had before. To make your files work again, just put the appropriate extensioon behind its name again.