Check to see if you have a burr in the stitch plate, this happens sometimes when you break a needle, you can use an emery board to file it off.
Or a burr in the hook area...

And another thing to check--I had two needles suddenly SNAP right in the middle of sewing. When I figured it out I saw that I was using an old spool of thread which matched what I was sewing. As the thread was unwinding, it got caught in the little slice in the top of the spool where you used to catch a thread when hand sewing.

Always thread your machine with the presser foot up.

I had the same problem and just started all over. I changed the bobbin and needle and dusted for lint and rethreaded the machine. Something worked - I just don't know what it was.

Check your bobbin case to make sure there are no nicks on it that might be snagging the thread when stitching. Sometimes when you break a needle it damages the bobbin case. If there are nicks, take some very fine sandpaper and smooth out the nicks. If that doesn't fix the problem, you might need to take the machine to the shop and have the timing checked.