Lens Trouble

[roosterfist]

Some time ago, I was given a Sigma 70-300 macro as a gift. The auto focus seemed to "stick" from the start, but as a newborn shutterbug I can work without it for the learning experience. But recently the aperture is "sticking" after I take a shot. You hear the *click* and the viewfinder goes dark, not black, just dark. Everything I've looked up points to an aperture ring and/or arm. Is this an easy fix, or should I take it to a shop? If I have to take it in, would the work cost more than what the lens is worth? Or is there something more to it, such as the communication between the camera and the lens?

I know that this is titled "lens trouble" and I'm not supposed to post questions about lenses on this section of the forum, but I'm unsure if it's just the lens, or a camera problem as well.