Thank you, Trekkie and Lucifer, for your suggestions and help.  I use XP Media Center, so it wasn't a Win7 or Vista glitch (which might have been easier to fix?).   After opening/closing Poser 7 several times and  rebooting the computer numerous times I just did a complete reinstall of Poser.   Very relieved that all content and folders are back in place.  I still, however, have no idea what initially caused the problem or why the content showed in Windows Explorer but the props folder remained grayed out in the program.