Usually this is just a matter of camera settings and/or lighting. Start by going into the Cinematography workspace and adjusting the camera settings. The most important are the three sliders for Shutter Speed, Aperture and ISO. (If you are using a manual lens, the Aperture slider will be grayed out.)

• Make sure the lens cap is off.  ;)
• Increase the shutter speed (to at least 1″).
• Open the aperture (go to a lower number). If you have a manual lens then open it up all the way.
• Increase the ISO.
• Increase the amount of light in the room.

If the live view remains black, send in a Camera Test through the Help menu.