I checked images online to have a look at the machine. Looks very much like a mechanical machine, which means you can use a screwdriver to take the top off. Do you have the manual for the machine? Typically mechanical machines need regular oiling, and the manual will show you all of the oiling points. I see that this machine has several defined-width zigzag stitch choices. I assume none of them work? You may be able to see what the problem is once you get the cover off. I would turn the wheel (towards you) to see what the mechanism is doing.