I am refreshly a beginner so I don't have much experience.
Problem: Just now I was editing a video together and I double clicked on the screen which has cause this black screen to appear and my work to disappear. Help.
You are looking at the footage panel and the CTI (current time indicator) is positioned before the layer even starts. There is nothing wrong with AE. The only problem is that you have not spent enough time learning what the UI is or how to use it. Spend some time in the Learn Workspace and check out the User Guide.