I had this problem too this morning. Only the first 7 (or 8) lines from the header are in the source from the page.
In another topic there was a reference to the theme files. I didn't make any modifications as far as I know but I removed the theme folder completely and copy a fresh folder from the web download. Problem solved.
I don't know what the problem was, but maybe I made a modification in the past and can't remember that... So for me it was theme related.