I don't know if your problem is similar to mine. I have a MacBook air that is 5 months old. Today when I went to turn it on all I got was a grey/black screen, the opening sound could be heard and the apple on the outer cover was lit up but nothing on screen. Called tech support and they had me do a few things to it and finally it turned on. I was told it was an issue with "kinks" with the new OS yosemite. Is this what you mean when it won't turn on? And did you download Yosemite? What finally made it turn on was turning on the power and immediately holding down "Option, Command, P and R". It blinked a few times but finally turned on. Since my is still in warranty, I am taking it in to the Genius Bar to have it looked at. Still goes to grey/black screen when it goes to sleep but keys do not wake it, have to close the top and then reopen. They, of course, said that shouldn't be happening and again blamed Yosemite and said they are working on a fix.