My Macbook Pro (2012) Screen suddenly Turns off while working.

My Macbook Pro (2012) Screen Turns Off randomly ,While working and keyboard back-light remains on ,and I have to Shut it Down (by Holding Power-button ), after that, my mbp wont turn on when i click power button (tried many times) , but after 30 min or 1 hr i can turn it on . why ?

( i've reseted NVRAM - PRAM and smc but it couldn't help ) and used Apple hardware test ( no Problem Found )

any idea?

Macbook Pro , OS X Yosemite 10.10.3b