Contact support --
http://www.guildwars.com/support/ -- with the subject "FORGOT USER NAME."
Supply any information about the account identifying yourself as the owner: That you're pretty sure your username was a hotmail address, the CD keys, the character name(s), etc. It might take a few days or more, but you will get a response and they should be able to help you.
Your username doesn't actually have to be a working email address, so don't worry about that. However, for your own security, once you know what your hotmail/GW username was, you might consider re-registering the email account so no one else can.
Good luck! Welcome back!