Jammed controls have been the root cause of many accidents involving GA planes and gliders. If you're lucky and the control is either jammed in (roughly) the neutral position or you can overpower what is causing the jamming rather quickly than you can either use the remaining controls (+power) to substitute for the jammed control (in the former case) or turn it into a non event that you walk away from although with a good scare (in the latter).
Any control that is permanently jammed with a significant deflection will get the plane in an unrecoverable attitude pretty quickly and then there won't be much that you can do if you don't have a parachute (either ballistic or personal).
Ciao,
Dg800
Edit to add: I'm of course referring to the primary controls only, as by the thread title I assume that is what you are referring to. Stuck flaps (in any position) or even a stuck throttle will lead to an emergency but usually not an unrecoverable one, i.e. you can manage to get the plane on the ground in one piece.
Last edited by Dg800; 14th June 2012 at 13:01.