70-300DL Macro Super autofocus problem

Question

My Sigma 70-300 autofocus has stopped working on my Canon EOS 3000. There is a little buzz as it seems to try to work, but nothing happens. The main 28-90 Canon autofocus lens is still working OK. I've tried cleaning the contacts on both the lens and the camera but no go. The manual focus works OK still. Any advice? It's several years old now and I mainly use a digital camera now but it's a pity - a good lens.