Make sure the thread path is clear and that the thread hasn't wound around the spool pin. On my Bernina I have a dial to adjust the foot pressure, do you have one? In my old machine the thread had cut a little trough where it was supposed to go through one of the thread guides. Every time a little thread bump caught in the slit it broke the thread and sometimes the needle. I fixed it by bypassing that thread guide and taped a little paper clip to the machine for a thread guide instead.