Rear internal hub won't shift gears I have a 1963 Sears cruiser, three speed, which I bought very recently. On my third ride, the middle speed stopped working, and went loose; there wasn't any "traction" when I pedaled. It happened as I rode in a particularly hilly part of town. I'm new to biking, so perhaps I shifted gears at the wrong time? The highest and lowest gears still work fine, only the middle speed doesn't work. 
Before buying this bike, the mechanic realized that the lowest gear had this same problem, and fixed it pretty quickly. Do I simply need to tighten or loosen the changing mechanism? Or do I need to replace it?

Thanks,
Have you tried changing the tension on the shifting cable? There's usually a tension adjuster on the outside of the hub. It's unlikely to hurt, and it's easy to do. I'd tighten a little at a time until all the gears work well. 

If that doesn't do it, you may need to have the hub looked at. There could be some dirt inside the hub (it happens, even with "sealed" units.) Or it could be a mechanical issue; getting an internally geared hub fixed can take time and money. If you end up doing this, I'd look for a shop that knows what it's doing; it can be a little tricky.