I have recently acquired a Sigma 70-200mm f/2.8 (Canon mount, relatively old), and it focuses perfectly in auto mode, but in manual mode the focus ring simply won't budge at all. What could be causing this and could it be fixed?
Most likely the linkage between the focus ring and the focus elements is damaged. The link to the AF motors is likely fine. It should be possible to repair, but it would largely depend on if parts are available. The best bet would probably be to contact Sigma and see if they still have parts. You may have to send the lens in to get it serviced though.