I had an instance of a "no boot at the gray screen", and it was nothing more than a corrupted Finder prefs plist file.
Move it to the trash, reboot, and… back to normal.
However, you have to have a "second boot source" to find and manipulate this file.
That's a heck of a lot easier than doing a "clean install" via the recovery partition.
This is why I recommend that EVERYONE maintain a bootable, external cloned drive. Can be 'way more useful in a "can't boot" situation than a dozen Time Machine backups….
(Aside: I suppose moving a particular file can be done using terminal while in the recovery partition, but I don't know the commands, and doubt that too many would be able to do this in a "moment of extreme need" in any case...)
