Macbook Pro will not turn on, progress bar gets half way and turns off

I have a macbook pro that Would randomly shut off. Now it will not turn on. When I try to turn it on the Apple logo will come up and the bar thing will get half way there and freeze, go to a gray screen and nothing will happen.... Sometimes it will shut it's self off and try to turn back on... I have all my school work on there and I need it for class! Please help!

I have OS X Yosemite... It is a 15 inch 2012 Macbook Pro...