If no images are displayed in the screen window, this may have various reasons:
Solutions:
Screen and screen field are not linked up with the tracks in the timeline.
Solution: In the Track Properties, check whether the tracks are linked up with the appropriate screen field. See Track Properties.
Your images and videos are located on a CD/DVD or on a hard disk requiring a write-permission.
Solution: Copy all project media onto a hard disk you are holding a write-permission for. Search for the files if the path has changed. See Finding missing files.
If this doesn't help eliminating the problem, your graphics card may be unsuitable for hardware rendering. See also Graphics card.