Hey! I was just trying to troubleshoot this same issue and, though you may have gotten an answer already, I wanted to provide on just in case and for anyone else who stumbles across this post! The simple solution is to open up the minecraft launcher and just before you actually launch the game, simply turn off your WiFi. It should say 'Play Offline' when you go to hit the launch button and that's exactly what you want! After that the game should load pretty easily(assuming your computer can handle it), and from there you can simply turn your wifi back on or leave it off. Hope this helps!