I have a MacBook Pro Mid-2011 and I decided to install OSX Yosemite yesterday, everything installed just fine and then it restarted it's self as normal but when it is booting up it is stuck on the white screen with the apple logo and progress at about 50%. It's been stuck like that for almost 20 hours now and I don't know how to fix it! I've heard things about putting the Mac into recovery mode, (Command R) but I didn't have a back up of any of my files with Time Machine or an external hardrive. Does anyone have any ideas? Thanks