Buildings typically represent the physical body in dreams. That you were in the top floor shows that your awareness starts intellectually, in your brain. It's also where things start to go wrong. Well, sort of. It isn't until the elevator starts to go down that you follow your intuition, and by golly you were right! Something just below your neck is going wrong, and destroying the rest of you in the process. I'd almost want to call it the face...which would mean that it has to do with how you view and interact with the rest of the world. But in following your intuition, you gain a higher perspective, reflected in the bird's eye view.

I also have to agree with Mike - what is this significance with the number 20? Only you can answer that.