've recently installed the latest Yosemite update and after restarting my computer it wouldn't boot again. The bar only fills up about a quarter of the way after log in and then my computer shuts down. I have tried disk repair and I have also tried reinstalling Yosemite but I get an error message every time I try. I can use guest mode but that is all. There was absolutely no problems with my Mac before the update. Please help?