Find your complete model number, and type that into the web-site: "searspartsdirect" and you will be presented a picture with schematics, and also the ability to obtain a manual.
God bless your efforts.
Most likely it is seizing due to old oil that has dried. Avoid trying to use it until you get it serviced or the motor may be damaged. Your sewing machine needs periodic oiling and cleaning in order to run smoothly, like your car.