Sometimes an impact to a rim does not turn it into a full "potato chip" shape, but only bends part of the rim sideways. A single "blip" which can only be removed the same way it got there. Violently.

NOTE: This is an emergency repair only and should in no way be considered permanent. As soon as you get home, start looking for a new rim!

Since it was a sideways impact that bent the rim, it is going to take a sideways impact in the opposite direction to straighten it. First, take the wheel out of the frame and find the bent part. Don't remove or deflate the tire unless it went flat. If it is flat, fix it and inflate to full pressure. This is important.

Found it? Good. Loosen the spokes in the area of the bend about 1 or 2 full turns.

Now hold the wheel so that the bend is bent in a downwards direction. Turn it so that the bent section is farthest from your body (pointed downwards!) and hold the wheel firmly with both hands.

Raise the wheel above your head and swing it down against the pavement, hard! Yes, you read right! you have to hit it at least as hard as the hit which knocked it out of alignment in order to get it back reasonably straight. Sometimes a good, barbarian yell helps.

Be sure it hits the pavement in the center of the bent section.

Now check your work by holding the ends of the axles and spinning the wheel while looking at the rim. It won't be perfect, of course. It never will be perfect again. But try to get it as close as you can. If the first hit didn't straighten it enough, hit it again. If you bent it back too far, hit it again, on the opposite side this time, and not as hard.

When you have "forged" it back reasonably straight, tighten up the spokes you loosened the same number of turns, put it back into the frane and true it as best as you can.