So I spent 4 hours creating this animation and I started rendering it, after an hour, it was on frame 20. While it was rendering I was playing a game and then the screen froze. So I waited 30 minutes for it to unfreeze. But it didn't unfreeze, so I turned off my computer, I went back to blender and it stopped rendering, then another hour passed and I realized I didn't have Ambient Occlusion on, so I Re-Rendered the video, then after 6 hours I realized that the sky disappeared, so I put in a new sky and Re-Rendered but then I realized I didn't save half my work and after frame 300 it was just still. How can I Re-cover my animation?
The resources of your computer are limited
Rendering and using your computer for something else at the same time is not a good idea, not only will the rendering go slower, but you might reach the limits of your processing power and memory, and crash your system when you run out of resources. Let the computer alone when you render, while you do something else. Pick up a book or go for a walk or do something that doesn't require a screen...
Besides that. Never Ever Render Animations Directly as Video. Render as an image sequence first and encode later to a video format. Not only will you be able to pause and resume the render (so that you don't have to break the addiction to your favorite game, or to prevent the rendering from bugging down the performance of your game), but youi'll be able to try compression schemes so that your video has better quality.
Can you recover something from the crash? Probably not... but explore the following links: