Need to Back Up a MacBook that Won't Turn On I have a Retina MacBook Pro running El Capitan. Recently, the machine refuses to let me log in. After booting and entering my password, the Mac starts loading, but freezes once the loading bar is full.

With Apple Support, I tried booting in several different modes, restarting things, etc. I was unable to login or see anything other than loading screens. So, they had me book an appointment for the Genius Bar.

I am wondering if there is anyway to back up a Mac in this state. I have Googled and searched this forum, but all the methods require some functionality on the device that needs to be backed up. If I can't back it up, so be it, I just wanted to check as it has some very important files.
Can you restart into Recovery HD? If you can, then you should be able to copy the HD onto an external HD using Disk Utility.

If you can't do that, but have another Mac available, you can restart into Target Disk Mode, and connect the two Macs together using a Thunderbolt cable. On the other Mac, your HD will show up as an external HD, and you'll be able to back it up.