Sounds to me like it could be a faulty hard drive.
Have you tried restoring from a Time Machine backup??
Also, do you know what version of OS X is on the MacBook Pro? This information could get you more help possibly.
Have you tried this though??
If it is
OS X 10.7 Lion:
1. Boot your Mac and hold down ⌘-R (Command R) to boot from the Macs Recovery HD partition.
Note: You can also boot from a 10.7 installer drive , boot to Target Disk Mode and connect it via Firewire or Thunderbolt to another Mac, or use some other 10.7-booting drive. As long as you have 10.7′s Disk Utility, this should work.
2. Open Disk Utility.
3. Select your locked hard drive.
4. Under the File menu, select Unlock Drive Name or Turn Off Encryption
, based on what you want to do.
5. When prompted for a password, you can enter the password of any authorized account on the drive.
6. If you unlock the drive, you should then be able to use Disk Utilitys repair tools to hopefully fix the problem thats preventing your Mac from booting.
7. If you turn off the encryption, the encrypted drive will decrypt. Once its finished decrypting, you should be able to access your data again using normal recovery methods (booting from another 10.6 or 10.7 boot drive, utility drive, etc.)
If it is
OS X Mountain Lion 10.8.4 or later:
1. Boot your Mac and hold down ⌘-R (Command R) to boot from the Macs Recovery HD partition.
Note: You can also boot from a 10.8.4 installer drive , boot to Target Disk Mode and connect it via Firewire or Thunderbolt to another Mac, or use some other 10.8.4-booting drive. As long as you have 10.8.4′s Disk Utility, this should work.
2. Open Disk Utility.
3. Select your locked hard drive.
4. Under the File menu, select Unlock Drive Name
5. When prompted for a password, you can enter the password of any authorized account on the drive.
6. Once you unlock the disk, hold down the Option key on your keyboard and click on the File menu.
7. Under the File menu, select Turn Off Encryption
(with the Option key held down, its no longer grayed-out.)
8. When prompted for a password, you can enter the password of any authorized account on the drive.
9. Disk Utility should briefly display a progress window labeled Starting conversion to JHFS+
Your drive should now start decrypting.
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Attempts to restore the. OS reports that the boot disk is locked.
How do I unlock that drive?
.