Alternator not spinning…

I replaced an alternator on a tractor around a year ago. I was working the tractor yesterday when the battery warning light came on. When I looked, I noticed the alternator was spinning very slowly. There was no noise or burning smell, so I continued working for a while longer. I had another look when I finished, and by then it had stopped spinning altogether. Again, no noises or smells. I had no time since to have a look, but would anyone have any idea what it might be? I’d assume a failing bearing would make a noise? The tractor is used regularly, so I don’t think it’s seized from lack of use.