If you changed the sort mode at the top to something that doesn't support alphabetic sorting, then the alphabet list won't be shown. For example, if you changed it to "Date Created", then the alphabet list doesn't make sense in that situation, so it is hidden. If you change the sort mode back to A-Z, then it should show up again. If the sort mode changed on its own... that is very strange indeed but I have heard of some configuration files being modified during the update when they shouldn't be for some users. I've never encountered that myself, but it does seem like it happens occasionally to users.
Mike
Mike