I host a weekly Minecraft night. All LAN server. Tonight, the Minecraft authentication server was down for nearly an hour. Blew game night out of the water. There used to be a "play offline" ability, but now, when I start the launcher, it immediately goes to username and password, and even left blank, I can't get past that screen. Any way to play LAN multiplayer games offline?
Your computer is trying to connect to mojang servers, but is failing, and is stuck in a retry loop. You should disconnect from the internet, start minecraft in offline mode, and then reconnect to the internet.
There are other to accomplish this, but they're all more complicated than you need. You could blacklist the mojang servers in your lan router, or your HOSTS file, like Aragón's suggested, but that only makes sense if you absolutely have to use other internet programs while minecraft is still online. (or planning on not having a valid connection to the mojang authentication servers, which there is no legitimate reason to do, but....)