[Foobarbob] posted a pretty simple job of bringing an LCD viewfinder back to life. He doesn’t specify what was wrong, but since he replaced the backlight, we’re guessing that was the main issue.
The generic, non-branded gadget slides into a hot shoe or cold shoe and uses mirrors to bounce the view to the top of the camera, rather than relying on a standard viewfinder or LCD screen.
See DSLR. Small, point-and-shoot cameras use a "live preview" LCD screen to take pictures and display the stored images. Digital SLRs have a traditional viewfinder that you press your eye against ...