How do I fix flattened carpet pile? Can I prevent this from happening?
Lay a very damp, but not dripping, heavy towel over the mark, then iron it with a regular clothes iron on high heat.  Go over it for a few minutes until it's good and steamy.  Then remove the towel and brush the carpet with a stiff brush, against the direction in which the fibers are lying down.  Try to get them raised as much as possible and then let it dry.