I think most of Video Copilot's plugins use your GPU and a black screen in the composition window is often associated with a "soft crash" of the plugin. There's a lot of layers in your comp, if a lot are using Saber, maybe it is more than your GPU can handle.
It might be worth restarting your computer and see if the problem returns and if it does, try turning off a few layers / effects as Ishan suggests (you might need to restart again if I'm right about the GPU crashing.)
This shouldn't be happening. Could you let us know your current After Effects version (Help > About After Effects) and system specifications (OS, OS version, CPU, GPU, RAM)? Do you only see this issue while using the Saber plugin? Does disabling the effects temporarily bring back the preview? We're here to help.
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