I'm having the same problem. I'm repairing a water damaged phone, and after the cleaning, it would hang on the galaxy S2 logo. I got it into download mode, and left it that way while I was Dling the proper Odin files, but when I came back 15 minutes later, it was a black screen. I finally realized it was still working, and navigated into download mode by memory, and successfully flashed the firmware. But now the screen that was working, refuses to turn on, although the capacitive lights do, and the phone is seemingly working normally.
It seems like it has to be something simple, the LCD shouldn't just die altogether for no reason. I'll keep messing around, and if I figure anything out, I'll report back. Or maybe someone else figures it out, and I can benefit!