Q: Apple MacBook Pro 2012 fails to boot

What happens when you boot to Recovery

OS X: About OS X Recovery - Apple Support

and repair/Disk Aid the startup disk via Disk Utility that is in Recovery?

If it is repairable then reinstall OSX like the article says

That is also covered here:

About the screens you see when your Mac starts up - Apple Support

This is the Mac Pro desktop forum. I requested your post be moved to the MacBook Pro laptop forum.

Repair Disk refers to the internal hard drive, not a disc drive.

Do as Illaass suggests and boot into the Recovery Drive. Turn the computer off, then turn it on and hold Command-R until you see the Apple logo.

Select Disk Utility, then select your hard drive (called Macintosh HD by default). Depending on your computer's operating system, you will either click First Aid, then Repair Disk, OR simply select First Aid, and it will ask if you want to repair the disk.

When it's finished repairing the drive, it should say if the disk was repaired successfully or not.

If your MBP is a retina model, the 'disk' is analogous to the flash storage in your MBP.  Run the repair.

Ciao.