Potbound is a term meaning the plant is in too small of a pot for its roots. If the roots don't have enough room, they will grow around and around until they form a tight ball the size and shape of the pot. The roots can get so tight that some of them choke off others. Also, when the pot is too small and is filled with roots, the roots are unable to absorb moisture. Although root bound plants usually tend to wilt or die on all sides, I have seen them die on only one side, as you describe. That isn't necessarily the cause of your plant's problem, but the first thing to do is to gently remove the plant from its pot and look at the root ball. Do the roots look healthy all around the ball? Do they wrap around and around the root ball, forming a solid hard mass, or can you gently crumble a little of the soil away? Does the soil smell fresh and earthy, or does it smell sour? If the roots and soil look okay, then gently replace the plant in its pot. If it's root bound, you need to get a pot one to two sizes larger and repot the plant.
Next, it's time to examine the leaves. Look carefully at the tops and undersides of all the leaves, and also examine the stems. Do you see anything that looks like little dark spots, that might be bugs of some sort? They don't necessarily need to move around. Scale insects, for instance, suck the juices out of plants, and they are immobile in the adult stage. They look like little brown bumps on the stems and leaves. You can use Google Images to find pictures if you like. If you see fine white webs on your plant, then it may have spider mites. There is a lot of information on this site about how to treat various insect infestations.
If you can take pictures of your plant and post them, then others may be able to offer better help. What I've given you here is really just the basics.