You are not able to boot the computer at all?
Or... can you boot into recovery mode?
If not...
Do you have access to ANOTHER Mac that IS working?
The following is something you could do -- IF you can boot into recovery mode:
1. You'll need a USB flash drive at least 16gb in size, or a USB external hard drive of some kind
2. Boot into recovery mode
3. Connect the USB flashdrive or external drive and open Disk Utility
4. ERASE the external drive to Mac OS extended with journaling enabled.
5. Quit Disk Utility and open the OS installer
6. Choose to install, BUT -- "aim" the installer at the EXTERNAL drive, NOT at your internal drive.
7. See if you can install a fresh copy of the OS onto the external drive.
8. Does this work? Does the installer finish the job?
9. If so, begin the setup procedure and create an account for yourself, so you can boot FROM THE EXTERNAL DRIVE for the purposes of working on your INTERNAL drive.
TIP:
To boot from the external drive, do this:
1. Press the power on button
2. IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears
3. Select the external drive with the point and hit return.
If you can do this, come back and report, and I'll tell you what to do next.