Update: I tried holding down the small button on the front of the control board while powering the unit on, and it came to life now and booted normally. I don't know if the button press did it, or if it was just luck. I'm afraid to touch it again now.
good to know.
I have a bad unit that is very prone to this kind of behaviour: it simply want to boot after been switched off.
the only way to resurrect it is reflashing the fw through the SD card.
next time i'll give this method a try. Did you try it by luck or did you read somewhere, maybe on some BBB forums?
Just luck/guesswork.