Are you talking about a Kerbal who has become unresponsive while in a command chair, or on the surface of a world? Both are a bit of a problem. If there is another Kerbal on the surface, have him/her go over and run into the paralyzed one. This will often reset it. If there is no helper, try turning on the RCS pack with the R key, and try to jet the Kerbal loose. If that doesn't work, your only options are loading a save or sending a rescue mission.
If in a command chair, that's an unusual bug which I myself have not experienced, and I do not recall what the fix might be, if there is one.