Just yesterday, I changed the name of one of my Packs. Now today, I can't sign in on that account! Have I lost my Pack forever? Please help!

There should be a 'I can't login' button or something along those lines, right next to the login bars. Press that and enter the email you signed up to that account with, and WP will send you an automated email with your pack name and password.

Well if you changed the pack name then your new log-in is the new pack name :)