Disclaimer: If you are reading this, chances are there is a hardware problem with your Mac. In my case it was a faulty logic board, which had to be replaced.
Use this guide to get your Mac up and running again and create a full backup of you system as soon as possible. If the problem repeats itself, I recommend you take your Mac back to Apple for a check-up.
Today I was happily working on some Java code, when I decided to relocate to a sunny spot in the backyard. I closed my MacBook Pro, walked outside and opened my MBP again to continue work: a black screen!
The screen of my Mac stayed black, although it did not indicate to be in a sleeping state. WTF? I restarted the Mac, removed the battery, reset PRAM/NVRAM and the PMU, but nothing worked! ARGH! Then, I found a solution that worked.