MacBook Pro black screen - or reset password
This happened to me too with my macbook pro 15 (mid 2012) running yosemite. What I did is took a flashlight and see if there was an image still on the screen. What I found out is that I was able to see the log in screen but it was all dark. After logging in all was back to normal. 

Eventually if that doesn't work you can reset the pram of your computer. Follow this link http://support.apple.com/en-us/ht3964 .