Updated Yosemite -> now in grey screen when booting

I have downloaded the latest update of Yosemite today. After the download was completed my mac book air was supposed to restart, unfortunately it only took me a completely grey screen (no question mark, no apple logo, no install bar).

I have followed all directions I could find on here:

- normal restart (keep the power button pressed for longer) - no result

- tried to start in safe mode with SHIFT but didn't work

- tried the internet recovery mode (boot with COMMAND + OPTIONS + R simultaneously), didn't help either

- I am not using McAfee which has been mentioned as one potential source for the issue

- PRAM reset doesn't work either

- tried COMMAND + OPTION + SHIFT + ESC to get around the frozen grey screen, no luck

does anybody has any similar experiences or any solutions?