Depends. There are actually two different possibilities.
If you just came down hard on the side of your rim, you probably bent the rim. If that's the case, the spokes on the side that the rim is bendt toward will be loose. You can try to pull it back into line by tightening the spokes on the opposite side, but you'll never make a good wheel out of it. Eventually you are going to have to replace the wheel.
Sometimes that kind of incident will pull the rim "over center". When that happens, the spokes pull the rim into a potato chip shape. That's fixable provided that you don't try to ride on it too much first. Some guys just bang the rim on a work bench or hold it against a wall and try to push it back "over center" into line again. My solution is to loosen every spoke until one thread shows and hope that the rim springs back into line. In either case, you need to retension the wheel a little more or it'll do the same thing again sometime.