Randomly can not host a game

ok, so nothing has changed from one day to another. hell, i didnt even shut down my computer. i am on a school network, and have always been able to host games, then one day, i log on, and no one can join my games. i sit on the phone for a while with the school tech support. not a single thing has changed to the network (seeing as it was sat it was working, and sun it stopped working) Nothing has changed on my comp, nothing has changed on the network, the game has not downloaded anything, why cant i host a game? (it has always worked before, no firewalls, no nothing installed to stop it)