Thread breakage during sewing can be a real problem. Knowing what caused it can help you to prevent it from ever happening again.
Here is a checklist for trouble shooting thread breakage:
Damaged or incorrect needle: Change the needle, as it may have dulled from overuse or could have a burr or nick. If the needle is new, check that it is the correct size and the eye is large enough for the thread type being used.
Defective or old thread: Lower-quality threads may have thick and thin spots that lead to breakage Thread that is too old becomes dry and brittle as it ages, causing it to break easily. (So inheriting grandmas thread box may not be such a treasure after all)
Improperly threaded machine:Check to see that your spool is properly positioned on the sewing machine. the thread may be getting caught on the spool cap and as it comes off the top o the spool. The solution may be as simple as turning over the spool on the spool pin.Or, if the presser foot wasn't raised when you threaded th machine, the thread may not be caught between the tension discs inside the machine. the quickest fix is to remove the spool and re-thread your machine with the presser foot raised.
Operator error:Pushing or pulling on the fabric or allowing drag to be created by hanging a heavy quilt over your work surface can increase stress on the thread and cause breakage.
Tension too tight:Refer to your machine's manual for the proper tension setting.
Wrong thread: You may have the wrong thread type for the fabric you've chosen. Change the thread and sew on a scrap of fabric to see how a differnt thread performs.