I received a message saying "user was removed" that was accompanied by a change in reputation and I would like to see what was removed. How can I do that?
A 'removed user' means that somebody's account was deleted and the votes that they cast were taken away. To my knowledge you cannot see what specific questions and answers they voted on, the change in reputation is the net change in how many times they either voted up or down on your questions and answers. This is tangentially a duplicate question.
As an aside, there is an algorithm in place that makes it so if the user was a large contributor to the site, their votes are not removed because it would have a serious impact on some other users' reputations and so on. See the meta question linked in that other question. The point is, votes on some of your questions and answers were removed because someone deleted their account, or an account was deleted by the mods. It must have been a small amount of votes because if the user's account was very active, the votes they cast would have remained. I'm almost 100% sure there isn't a way to see what specific questions or answers the votes were cast on. You could try looking through your questions and answers and seeing if there are any that have a lower vote count than you remember.