Unfortunately, you can't pause an ongoing render, but for future reference there is a way to get similar functionality. You can render your video as a series of PNG images and then put them all together at the end.
First, in the render tab, go to output and make sure PNG is selected. Point it to an empty folder on a disk with plenty of space because it is going to spit out a lot of images.
Then when you click render animation it will make a PNG for each frame. When you stop the render, you can just set the start frame to the last frame it rendered and then continue.
Once you've rendered the whole thing, take all the images and put them together into your video. This guy does a pretty good job of explaining how: