You don't mention what brand of machine you have, but I had a similar incident with my Janome 6600. I tried everything to no avail. I finally turned off the machine and walked away. After calming down and returning, I turned on the machine, rethreaded top thread and bobbin thread.....tada! ....everything was good again. It seems that the thread came out of the tension discs, somehow. By turning the machine off and then on again, it reset.