El Capitan installation problem MacBook
i have 13 inch MacBook Pro retina and for the past 2 days I have been trying to upgrade to el capitan. I download the installer and the installation goes smooth, but once it finishes, it goes black and the Mac does not restart. Pressing any button makes the backlight go off however the screen os just black. I leave it like that for a while however it does not do anything. When I try to restart it manually the boot takes a very long time and once it gets done, the login appears with several flashing vertical lines and hust a great background eben though it's supposed to be my blurred background image. Typing my password takes a long time as it starts lagging a lot. Once I get logged in the computer is very laggy and unusable. What went wrong apple? how does this happen to a laptop like this?
iPhone 5s, iOS 8.1