I solved this problem in my macbook pro late 2011. First i did pram vram reset and smc reset with no success. Then i tried inserting a dvd and tried pressing eject on startup. I tried removing the Ram sticks and disconnecting the hard drive also. But nothing happened at all.
So i tried booting the mac and leaving it turned on with the black screen and the little blue led lit until the battery was completely off (i checked battery status clicking the little button on the left side). Then i put it on charge until the battery was fully charged again. Then i did an smc reset. Finally i just pressed the power button and it booted!
So i think in my case it could have been some problem with the internal power management.
Hope other people could fix this problem too.