Black screen on my computer but not my brothers

So about a week ago I was playing league perfectly fine on my three year old MacBook Pro, but then one game I ended up with just a black loading screen. I restarted my client over 10 times but I still couldn't get into my game so I restarted my computer and kept on restarting my client. Later that day I uninstalled League and downloaded it again but still even when I made a custom game with just me in it I got a black screen.(I actually downloaded league a second time later that day.) The next day I got a hold of my brothers slightly older MacBook Pro and played League on his computer just fine.(Actually in one custom game I made I played the reworked Shen {{champion:98}} and was completely invisible and couldn't move, but I haven't had this problem or any significant ones.) But why is this? We both have the same type of computer but his is a year or two older than mine. I've tried every couple days to make a custom game on my original computer but I still get the black screen immediately after finishing champion select. I'm also not even able to spectate games, I just end up with the black screen again. Any help is greatly appreciated. Thanks

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