Cross posted from a similar thread:
Ok I had this very same issue, but I got passed it and I have no Idea why. Heres what I did:
My 2011 MacBook Pro was stuck on the boot screen (Apple logo and progress bar) after a seemingly successful installation. The progress bar was stuck at around half way for 2 hours.
So I force power down and restart. Same thing happens again.
So I boot into my Windows 7 partition to find a solution. No answers are found other than to boot into the recovery disk and reinstall
I shut down Windows and boot into the recovery disk. I was about to hit reinstall when I realized that I need to back up a few files Ive been neglecting for a while. So I shut down the recovery disk without doing anything.
I then restart the computer but I forgot to hold down Option and it starts to boot into the the disk I tried to install Yosemite on, and IT WORKED! It booted into Yosemite normally and Ive not run into any further issues.
So it seems the solution to this problem is:
1) Force power down
2) Boot into Windows and putz around the Web for a while before shutting down.
3) Boot into the Recovery Drive and dont do anything, then shut down again.
4) Boot up normally and it works
5)
6) Profit!
This worked for me for what seems to be the very same issue as the OP, but I stumbled on to the solution by accident so YMMV.