Next to the event in your reputation list there is a learn more link that says:
This message means that a user who voted for one of your posts had their account deleted (either by request or due to violating the network's terms of service). As a result, all of their votes were removed, and the reputation you gained or lost from them was undone. (...)
This removal occurs whenever a user is deleted, unless that user had a very high reputation score. Because high-reputation users have usually cast a great many votes, removing all of them could be that much more disruptive to other users.
There is also an FAQ post on Meta discussing it: What does “user was removed” mean and why did my reputation change because of it?
The user that was deleted did not have a high reputation score, so the reputation from the one up-vote they cast on your question is gone. If they had down-voted your question you would have seen some reputation added back to your account when the user was removed.