I've had that happen to me too. I make sure I drop the needle twice to make sure the threads lock. It always seems to work doing that. It can be fixed though, even without takin all the stitches out.
Thread a handneedle, put both ends through the eye. Bring the need up through the machine needle hole, catch the thread then back through the same hole, then pull the needle through the loop of thread on the other end of the needle two or three times. You'll have a very small knot that's barely visable on the backside.