As I think you have managed to work out (looking at the account history), a user can re-enable their account by requesting a new password from the login page, then logging in.
If they are no longer a member of your site then they will have to re-join. If the site's membership policy doesn't allow people to request to join the site then you as an administrator will have to invite them.
Once they are a member you can grant them Admin rights if you wish to.
If you get stuck let me know...
Joe - Voice Admin