roboh I've also come across a similar problem. I would often turn on my computer with nothing more than a black screen and the fan running. It would run pass the initial start up screen but that is it. I couldn't quite figure out what was wrong with it but I did notice it happened more often when I was using an external connector with my HDMI. Have you attempted to run your computer in safe mode, or does it not come up with any display at all?
My issue has been alleviated for now as this happened about a month or two ago. I cold restarted it (pressing and holding the power button on the black screen) and after a few trys an automated disk error repair screen popped up. This took about 1-2.5 hrs or more (I didn't time it). Since then it has been running normally.
