Originally Posted by
Lyncat
Make sure the thread coming off the spool hasn't gotten itself wrapped around the spool pin under the spool.
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.