Did the motor slip down and change the tension on the belt? Is the motor attached firmly or does it move? This wouldn't have made a "bang" I don't think, but I would check that. I would also take a peek at the gears at the top and the bottom of the machine to see what's doing with them. I would also unscrew the nose plate on the machine and have a peek at the needle bar and the connecting rods to see whether everything is still connected properly. I would be looking at whether somehow a screw came loose. Please note that the big screw that connects the counter balanced wheel is a right hand thread so it loosens the way that other screws tighten.