Hi, I hope i can help you.Try the following solutions and let me know if that works or not...I know you already zapped the PRAM and NVRAM, but do it again.Reset PRAM and NVRAM
Restart in Safe mode Make sure that the computer is shut down. Press the power button. Immediately after you hear the startup tone, press and hold the Shift key. Release the Shift key when you see the gray Apple and progress indicator. Let the Mac boot up...Run Disk Utility in Single User ModeTurn your Mac off. Turn it back on and this time hold down the Command (Apple) key and the S keys at the same time. Then type the following command.../sbin/fsck -fy (note the space between fsck and -fy)If you can't run /sbin/fsk - fy then type only fsk - fyHit return and the disk utility should run and let it finish. When it finishes and there is no error, then type reboot and hit return. Let the Mac boot up.
It only takes 1-5 minutes to erase the drive. I think your hard drive is dying, that's why your Mac can't read it. Anyway,try again and if it didn't didn't worked, then you have to take your Mac to the Apple store to fix/replace the hard drive.Best Regards,