It is an unambiguous violation of ethics for a collaborator to be dropped from the list of authors.
If you wrote a significant contribution to the paper, you are in the right. Your supervisor has no ground to stand on. You should ask your supervisor (don't accuse!) if he/she has included you as a co-author. If a mistake has been made, the sooner it's fixed the better. You could also have a good claim to first authorship, though that is something to decide between authors.
Edit: a little humour - full credit goes to Nik Papageorgiou / The Upturned Microscope.