Help

Good Day,

During updating my macbook pro retina 15in to OSX Yosemite 10.10.4 the computer went black screen and now my computer giving my hard time starting up.. the screen remain black.. SO i have to hard-shut the computer and start again.

When the computer is finally on.. during using the computer, the screen sometimes goes black by it self (Keyboard remain lighted)

Please advise how to fix this.

Thanks