I restarted my macbook pro this morning and it started up with a chime and now is stuck on the grey screen. I have tries holding any different key combinations, none of them work, except resetting the pram, which does cause the computer to restart.
I have tried option, r, s, d, command shift v, command v, shift (before and after the chime), command option d r.
FYI I have been running yosemitr for a while with no problems. I shut my computer last night and it was working fine. This happened when I turned it on today.