MacBook Pro 2012 HD locks on boot
I got the machine to boot by starting up in recovery mode and turning off encryption on the drive in mention. Not sure what the implications of not being encrypted is though...

useful links:

https://support.apple.com/en-us/HT201255

https://derflounder.wordpress.com/2011/11/23/using-disk-utility-to-unlock-or-decrypt-your-filevault-2-encrypted-boot-drive/