It all depends.  It might have been a software glitch or an intermittent hardware failure.  A possible software glitch might have been Windows thought you were connected to an external monitor, but closing and reopening reset it.  A hardware problem may be a loose connection or a short circuit in the wires connecting the motherboard to the LCD.

Do some heavy Google searching and see if others have experienced the same problem.  A few months ago my wife's Macbook stopped recognizing the MagSafe power cord.  The Google searches said it might be a bad cord (nope: tried with two cords), a firmware issue (upgrade & reset procedure failed), or a motherboard failure (confirmed by a repair shop).  Fortunately I'd gotten in writing (email) that the repair shop wouldn't charge unless they fixed it (they probably hoped it was a firmware problem) and I sold it for parts on eBay.

I've also had problems watching Flash videos one or two months ago... the whole CPU would intermittently stop, then start again for a split second, then stop again. Sometimes it would never start again so I'd have to switch the PC off. But it fixed itself by now somehow... I guess they introduced a bug while updating the Flash player which I have on auto-update, happening quite often. Maybe you have a similar problem which will fix itself in a subsequent Flash player update...

That would explain why the problem didn't happen since the last time. I didn't remember if I updated the flash before the problem occurs.