Apple Macbook Pro Late 2011 freezing screen on startup
Do you notice any colorful graphics glitches? The startup screen would be filled with dots. The Late 2011 MacBook Pros are known for defective AMD GPU. The symptoms point to GPU issue. Apple has a free logicboard replace program. The program is available till Dec 31, 2016. Better you visit your nearest Apple Store or Authorized Apple Service Center for diagnostic. for more info about the replacement program, visit the link below.

Apple MacBook Pro Video Issues (Apple Support)