This machine gets pretty bad reviews on Amazon:
http://www.amazon.com/SINGER-3116-Si...dp/B0014MBN5E/
My bet is that the knob mechanism for selecting decorative stitches is broken. Possibly, depending on how the machine is made, you might be able to take the case apart and fix whatever is broken inside. This is what I would do with a vintage machine. However, many of the new cheaper machines have electronic components glued inside the case, so taking the case apart could ruin something else.