During the Yosemite Installation users are prompted to turn on FileVault, this has been causing some issues with some of our Macs. After the installation is complete the user is asked to login to unlock the drive, after they have correctly inputted their password the Mac will begin to boot and display the progress bar, unfortunately on some Macs no matter how long you leave it to boot, the progress bar will not progress further than 50%.
The easiest way to bring the Mac back to life is to reboot into Recovery Mode (using Command + R), open Disk Utility and from the File menu select Turn Off Encryption. For whatever reason while in Recovery Mode the disk will not start the decryption progress, now reboot the Mac into Target Disk Mode (hold down T when booting) and using either a Firewire or Thunderbolt cable, plug it into another Mac.
When plugging the dead Mac into the other you may be prompted for a password to Unlock the disk, use the users local password for this step. You can now check on the progress of the decryption from terminal by running the diskUtil cs list command. Once the Mac is fully decrypted you will now be able to unplug it and it should boot successfully.