Don't feel stupid. My ex-machine (long story) locked like that once and the problem was not the thread at all. You mentioned that it locked while you were doing FMQ. What happened to mine was that I was moving the fabric faster than the needle was moving and it messed up the timing of the machine. It can be fixed, but not by everyone. Modern machines are wonderful with all the bells and whistles, but with those comes a new set of problems. Hope you get your machine back soon.