MacBook Pro stops loading and turns itself off at login

Question

My MacBook Pro with OS X Yosemite won't load past the login.  It turns on, has me click on my login photo and enter my password, but then only loads about 50% and then shuts itself off.  Help!  I've restarted multiple times, and when it randomly loads with the apple logo, it will sometimes then route me to the option to reinstall Yosemite.  But it won't let me do that either.  it says my hard drive is locked.  One time, I was able to get to the Macintosh HD disk first aid screen and ran that.  It indicated that my disk needed to be repaired.  I clicked on "repair disk," and it goes through the motions, but I'm not sure what to do then.  If I restart, the issue at login persists. 

Answer 1

Reinstalling OS X Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Reinstalling OS X Without Erasing the Drive
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
Also see:
Reinstall OS X Without Erasing the Drive
Choose the version you have installed now:
OS X Yosemite- Reinstall OS X
     Note: You will need an active Internet connection. I suggest using Ethernet
                 if possible because it is three times faster than wireless.