In Lightroom 4, my thumbnails are not rendering. It shows that the images have been imported. It shows the metadata. But the images are grayed out. This is occurring on all of my collections and catalogs. I'm using Windows 7.
Most likely a corrupt monitor profile.
Re-profile it, or - just as a test - set the monitor to sRGB, restart Lr and see how things look.
Retrieving data ...