LoL freezing

hey guys, last week i picked my champion and everything was running smoothly until it is supposed to go to the loading screen. It just goes pitch black and it doesnt load. ive waited thirty minutes and finally i just had to turn off my mac. Yes, I am playing on a mac but its worked perfectly until last week. Ive tried to reinstall LoL and still it keep going back after champ select. Any suggestions?