Progress bar goes quarter of the way then black screen

Hi, I'm running Yosemite 10.10.5 and my computer was working fine one morning, then I turned it off to go out for the day. When I came home I tried to turn it on but it just loads the progress bar under the apple a quarter of the way, then changes colour weirdly (like darker grey, lighter grey) and then goes to a black screen where I can move my cursor around. I have tried several things more than once: resetting NVRAM, resetting SMC (does nothing I can see), typing in the fsck -fy and deleting some things I read somewhere, booting in safe mode (does nothing), repairing disk, repairing permissions, just plain turning it off and on, reinstalling OS X (errors), leaving it on the black screen for a while (brought up a log in screen and then black again), leaving it off for hours. Everything. Someone PLEASE help. Does this have to do with El Capitan??? I didn't even give permission to install, does it do it by itself? My MacBook is from 2013. Thank you.