We can't know. you have not provided any system info or other sensible details. And well, if you have been "working on it for days", then surely you should have multiple incremental saves and backups of it.
Mylenium is right. We need version number of the CC you're running, what you've got in your comps, and lots more information.
And, if you were working for days on it but don't have any incremental saves or auto-saves (or, ideally, both), why not? That is, like, rule 1 of computer work! I don't even work in a word processing program for longer than five minutes without hitting ctrl+s.
This has been happening to me as well. Usually during RAM preview. I'm on Ae CC2014. My current video is a demo reel, so the content is a bunch of mp4's with some text layers and a sound track.
The problem is that a blank window pops up, labelled After Effects, but with no visible content. When I try to close After Effects, an error message says, "the program is waiting for a response from you."
It's not the end of the world when it happens, but it certainly is annoying.