how can i fix my hob control knob?

when i turn the knob nothing is happening i.e the gas isnt been released. When i turn it, its not actually catching and is just loose. I have taken it off, and the shaft actually comes right out and all that was in the hole was a small spring.

When i pulled one of the other knobs off, which is working, just the plastic knob came off and the shaft remained in place fixed in its place.

Can i repair this? if so how?

david alexander
October 2011