Please help! I might fail this cl!?

I just did a 8 page essay on Gandhi that was half my grade. I found a source online that was an online book, so I read it and wrote it in my own words. I got it back with a zero, and my teacher said that I didn't acknowledge my source with a footnote or something like that. He said "You should acknowledge your sources whether quoting directly or summarizing the ideas of others, in which case you didn't so it's the same as plagiarism." I have a bibliography, is he still at right to do this?