Over the past two days, i've encountered something quite serious (IMO) with my Macbook. Basically, when I put the Macbook on sleep and then come back (lets say an hour later) - the Macbook's screen won't turn on although the fans are quite noticeably high (as in the sound is loud). I've tried pressing almost every button on my keyboard, yet no luck. I have to power the whole system down (by holding down the on/off switch). Any ideas as to what is happening? Apologies in advance if this has been discussed previously.