HI,
Some women don't get a true positive or they do only if they test very frequently (I am one of them). If you got a very strong line that the next day went to nothing I would take it that you had your LH surge on the day of that dark line. Therefore it would be expected that you would ovulate sometime between 12-36 hours after that dark line. I would keep getting jiggy with it for today and tomorrow to cover bases!
I wish you luck!