Singer, model 158.13170, serial 0025776?

I recently found my grandmother's Singer model 158.13170, serial 0025776 in her attic. I appears to be over 50 years old, unused from 25 years and after cleaning, oiling, etc., I need some help. The knob on the top which is the zig zag knob, won't function and it dosen't seem to snap into the groove desired for a longer and/or wider zig zag. Any suggestions?