If the Scene panel is completely black, even when no scene is open or when a scene with no puppets is open, the OpenGL capabilities of your graphics (display) driver might not be enough for Character Animator. Updating to the latest supported/qualified graphics driver for your machine's GPU might help. If your machine has a second attached display or video device, try detaching/disconnecting it, restarting the machine, relaunching Character Animator, then reattaching it.
If the Scene panel used to show your puppets but then stopped showing them, try the following:
If none of the above fixes the problem, please submit a bug report with as much detail as possible.
See also this related FAQ: FAQ: How do I diagnose and resolve an OpenGL-related error?