I was playing ACNL but I need to set my 3DS down. I set my 3DS down on my desk, the screen went black immediately and gave me some error message. It wasn't the error message of a removed gamecard either. I couldn't even press the home button to get to the menu. I had to shut it down.
I turned it back on and it seems to be ok now besides being laggy once in awhile..... but I don't know what happened.