Cannot find file after exporting on iMovie

When using iMovie, I keep trying to export my film via the "export movie" option. It usually takes around 12 hours because the project is 2 hours long and I have been trying to export it in the highest quality, 1080p. After it supposedly finishes exporting, I cannot find the file anywhere on my computer, but I know something must have been created because I was down 18GB of storage, which is the amount of space iMovie said it would take up. I tried searching for it on finder and in spotlight, as well as using advanced search options in finder to see if I could find it by the size of the file. I restarted my computer to see if that would work. Once I turned my computer back on, I could still not find it and I noticed I had gained back the 18GB worth of storage. This is has now happened twice and I have no idea why this happening. In case tis helps, I am using a Macbook Pro operating with Yosemite, and it was purchased in 2011. Any help help or thoughts would be greatly appreciated!