• The upper thread could be tangled.
  Remove the thread completely from the machine. 
  The correct size spool cap should be used for the size of the thread spool.
  Re-thread the machine according to the directions in the manual.

Please refer to "Upper threading" of "Preparation" category.

One Point Advice   Place a spool of thread on a spool pin

• Remove the bobbin and make sure the bobbin is threaded properly in the shuttle race. 
  Remove any loose threads or lint in the shuttle race.

Please refer to "Installing a bobbin" of "Preparation" category.

One Point Advice   When thread got tangled

• The tension could be too tight or too loose.
  Set the tension to the basic thread tension setting or adjust the tension manually.
  
  
• The combination of the needle size, thread size and fabric is incorrect.
  Be sure to use the correct size needle and thread for the type of fabric that you are sewing.
  (Example: Denim-size 16 needle and 40-50 heavy-duty thread )

One Point Advice   Combination of fabric, thread and needle