Gray screen immediately upon boot. Cannot enter into safe mode, and cannot show boot loader with the Alt key.
You could also try holding command-R to start up in recovery mode, option-command-R to start up in Internet recovery mode, or command-S to start up in Internet recovery mode. Or try using Recovery Disk Assistant to create a recovery partition on an external disk.

If you can somehow start up from the recovery system, one thing you can try is to choose the Reinstall OS X option. It downloads and installs a new copy of OS X over your existing installation and keeps user files and settings in place. It fixed a problem I had where my Mac got stuck during the startup process. (I don't remember what part it was though, and I was able to start up in single user mode and from the recovery partition.)

If nothing else works, you could try replacing the hard drive with an empty drive. Since your Mac has firmware support for Internet recovery mode, it would normally start downloading a disk image for the recovery system from Apple's servers, and then allow you to install a new copy of OS X on the empty drive.