Why am I getting "Access Denied" on /user/login? Not sure what happened with my Drupal site, but I can't log in :s

When I go to /user/login, /user, /?q=user, /admin, whatever I just get "Access Denied", never presented with a Login form. Server response is 403.

Drupal 6 site.
Probably your IP has been blocked as a potential source of flooding/DoS attack. This may happen if there were a lot of requests from the same IP. 

That happened to my company's site once when we rearranged our LAN structure so all intranet users could only access the site through proxy server, and proxy server IP was blocked by Drupal automatically.

You need to check flood table - there must be a record with your IP. Just delete that record or all records (DELETE from `flood`).