I had this problem this morning. I was using my MBP before going to bed and just left it running and open when I tucked in for the night. In the morning I came back to it and saw it was sleeping (as expected) but I could not wake it no matter what I did. I powered it down with the button, let it rest for a second and tried to power it up only to get stopped at the grey screen no matter what I tried. Two days before I had upgraded the hard drive and RAM so I thought for sure it was a hard drive problem since I had never experienced such a violent death with RAM before. Before I tore it all the way down to put the old drive back in it I swapped the factory RAM back in it and it worked fine. So, if you have any other RAM laying around you may try it.

Fortunate I had purchased the new RAM just two days earlier I exchanged the bad sticks for a new pair and all is fine again. Good luck.