Janome 3125 sewing machine help?
I have a Janome 3125 sewing machine, which has worked fine until today....The bobbin thread refuses to move, when I try to sew with it I get about half an inch, then won't sew any more. I have to wind the wheel backwards to get the needle out, then have to CUT the fabric free because about 20+ threads are sticking out, stuck inside the bobbin mechanism.
Does anyone know what is happening and what to do?

Remove the needle plate and clean the bobbin case and the area under and around the feed dogs.
Raise the presser foot every time you thread the machine.
This opens the tension and allows the thread to seat into the tension disks.
Not doing this will not allow the tension mechanism to work properly.
Be sure the bobbin thread is feeding from the bobbin in the correct direction and through the proper guides.
You may need to adjust the tension on your machine, especially if you are using a new fabric – if it’s stretchy or very delicate, you may need less tension.
Make sure your top thread is loaded correctly – if it’s missed any of its various go-through points, that can cause the machine to lock up. Also, double check that your bobbin is loaded correctly, I sometimes have the same problem if I’ve put it turning clockwise instead of counter-clockwise.