Basic thread Break Fix 
Follow this short procedure to clear a thread break.  (Note: this procedure does not address troubleshooting the cause of the break, only to teach how to re-thread and continue sewing.  Problem may re-occur without proper troubleshooting.
1. Press T.CUT to cut thread still attached to garment2.  Repair thread break by re-threading upper thread or replacing bobbin.  3.  Press or press and hold STOP to move sewing position to just before the break

 4.  Press START to continue sewing.

Follow this procedure to troubleshoot thread breaks:

 

1.  Bobbin . Check to see if the upper thread is actually broken.  If it isn’t, and the bobbin has not been changed for a while, check bobbin and replace.  A remote possibility may be that the thread break sensor is defective for that needle, but this is the least likely of the causes listed here.
2.  Thread Path.  If upper thread is actually broken, check carefully along the entire thread path to make sure that needle is threaded correctly.  Pull the  thread through the needle and ensure the thread pulls smoothly through the
entire thread path. There is an excellent diagram in your manual on how the thread should flow through the tensioners, grommets, etc from thread cone  down to the needle.
3. Design or thread. If the thread path appears correct and the thread seems to be pulling smoothly, and this still doesn’t solve the problem, check to see if the break seems to be occurring in either (a) the same point in a given design, [i.e. poor digitizing] or (b) with that particular thread [old or otherwise inferior thread will be more brittle than newer/quality thread].
4. Needle depth/proper needle orientation.  Did you know that the needle MUST be installed in a specific way? The long groove must be facing forward, and the scarf (cutout section behind the eye) must be facing directly backwards. Incorrect needle depth is also a cause (i.e. was the needle inserted all the way into place?  Does the depth appear correct compared to the position of the rest of the needles?).   Check your manual for the procedure to set needle depth for the ultimate procedure.  Also, check to ensure that the needle is entering the hole in the needle plate in the center or close  enough to the center that it isn’t physically contacting the needle. 
5.  Check for burred/jagged surfaces.  The normal wear of sewing operations can cause surfaces that the thread contacts to become damaged/jagged.  Check the following places.  Crocus (abrasive) cord or a very fine grit sanding cloth or sandpaper, which can be  purchased at any hardware store, can be used to polish away the burred/uneven surface..
           a.  Surface of the rotary hook (diagrams will follow)
 b.  Needle plate – ensure that the edges of the hole are smooth
c.  Damaged needle – when in doubt, replace the needle, inserting as shown in (4) above.