My reputation went down by 15 points. Is it because someone deleted one of their questions?
Update Jan. 22, 2012: Two aspects of the answer below are outdated.
- Disaccepted answers now do show up among viewable point changes. (This is noted in the linked question below, and the relevant meta.SO thread is here.)
- Users with 10,000 points or more who are not moderators can no longer find all deleted posts from the tools menu. They can find posts deleted recently by someone other than the post owner. For other cases they have to know where to look, which is particularly difficult for deleted questions where one would have to know the URL. A relevant meta.SO feature request is here.
That probably means that someone "disaccepted" your answer to one of their questions. There's no alert when this happens, so unless you have a small number of answers there's no easy way to find out which one it was. The question Identifying a “dis-accepted” answer and the questions linked to in my answer there ask for this to be changed.
is possible that would be possible for the question was to be deleted after your answer was disaccepted, if all answers to the question had a low vote total (1 or fewer I think, but I might be wrong). Moderators and users with 10,000 points or more are able to see deleted questions, so if you suspect that a question you answered has been deleted you could ask someone to check for you.
Edit: The first sentence in the second paragraph has been reworded in light of the fact that the question was not deleted.