The most likely cause is a failure in the manual-focus linkage between the focus ring and the internal focusing mechanism. Since autofocus still works, the AF motor side is probably functioning, but the parts that let the ring drive the focus group may be jammed, disconnected, or broken.

It may be repairable, but the key issue is parts availability—especially for an older lens. Your best next step is to contact Sigma service and ask whether they still support that specific version and have replacement parts. They may require you to send the lens in for inspection and repair.

Avoid forcing the ring, as that could cause more damage.