1- start the MBP holding down the 2 keys ->command->s
hold the keys until you seen a black screen with white text
2- type: fsck -fy (there's a space between k and -
3- wait, the computer will scan the hard drive searching for errors. Could take time to complete the task (few minutes).
4- when you get the text: <<The volume "hard disk name" appears to be OK>> the HD should be repaired if it's a software issue. If you get the text: <<System file has been modified>> redo #2 until you see only this text: <<The volume "hard disk name" appears to be OK>>
5- once the HD repaired, type: reboot to load the OS.
This is the same procedure as using Disk Utility to repair a system software issue.