Rear internal hub won't shift gears
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.