If the needle on your sewing machine stops moving up and down, it can be attributed to a number of different factors, including improper maintenance and gradual wear and tear. By going through a series of steps, you may be able to correct the problem.
Broken Belt
Identify the problem in order to correct it. First, listen to the motor when the floor pedal is depressed. If the motor is running, but the needle is not moving, it might be a broken belt. Unplug the machine, and remove the motor cover. Look at the pulleys and inspect the belt. If the belt is broken or friction burned, replace it.
Improper Setting
Many machines have a bobbin setting, so you can wind a bobbin quickly on a spindle. When the machine is in the bobbin setting, the needle will not move up and down. This is a safety feature. Make sure the machine is in sewing mode, rather than in bobbin winding mode. A foot in the up position may have a safety interlock as well, stopping the needle from moving.
No Power
If the machine is plugged in, but the motor does not spin when you depress the floor pedal, you are losing electricity somewhere. First, make sure the outlet is working. If the outlet is working, the problem is in the machine. Fixing an electrical problem requires specialized knowledge of sewing machine electrical systems, and the machine should be taken to a qualified service center.
No Upper Thread
Many sewing machines have a "kill switch" on the upper threading area, built in as a safety feature. If the upper thread runs out, the switch will not allow the needle to move up and down. inspect the upper thread area, and make sure the thread has not broken or run out.
Internal Broken Part
If the motor runs, listen closely for any sounds of internal rattling when the motor is running. If you hear any strange noises, an internal part has broken. Immediately turn off the machine. Repairing an internal broken part requires specialized knowledge of sewing machine mechanisms, and should be attended to by a qualified technician.