happened to me a few years back. I almost did a face plant when the hub stopped and the crank spun freely. It can really be dangerous. I asked folks here and what was highly recommended was to drop oil "into" the hub. I had an older system, freewheel/hub. I laid the bike on its side, added a few drops of oil to the end of the hub(center of the freewheel) and let it work it's way in. If memory serves I dropped oil because I had an old non repairable hub. It stopped freezing up and lasted another winter.