There are different possible reasons for an empty output.
First of all, what Plant Simulation version and maintenance pack are you using?
Were you recording only the 3D window (that's the setting "client area only" in the video settings dialog) or the whole Plant Simulation window?
Finally, it might be helpful to know what video codec you were using. You find this setting in the video settings dialog, too. The selection below shows what is currently selected.
That's a known problem that has been fixed in maintenance pack 12.0.2.
If you update to that pack, the problem should have vanished.