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First of all you need to clean the connectors on both the lens and the camera. Do this by using a pencil eraser being careful not to get bits in the camera or lens. If this doesn't clear the problem is it that auto focus doesn't function with any lens. If that is so then you need a camera repair. If auto focus is only non functional on the Sigma 170-500mm and you can manual focus then the probable problem may be a broken ribbon cable in the lens itself. This will need to be replaced. Usually a quick and relatively cheap job.
I have a lot of experiencie repairing Sigma lens.
In this zoom 150-500, the problem is located at the focus mechanism. This is a rear-focus lens so the focus mechanism is close to the bayonet.
It is not very hard to repair, but is better if you send to a Sigma Service Station.
Normally the mechanism is stuck due to a fall or strong punch.
How new is the lens? In or out of warranty? If out or warranty most likely solution is to find a Sigma authorized repair shop because your description sounds like the auto-focus motor gear has failed. If in warranty, check out Sigmaphoto.com for repair sites and also download the SIGMA_SERVICE_REQUEST.PDF. I've only had to deal with Sigma once on warranty matter and they were very helpful, provided UPS authorization for shipping on their nickel.