Mac book doesn't boot My Mac book pro 13.3 inch retina display August 2014, did not boot after a normal shut down. It gave sound on power on but after stays grey. I tried all options available on net CMD+C, C, SHIFT+CMD+V but no success.
Here is the set of Start up keys that you can try.

I would try the Safe mode first, followed by Apple Hardware Test.

If it does not boot in Safe mode and passes the Apple Hardware Test, use the OS X recovery "cmd+r" to restore your OS X installation.