When you put the key in, and turned the knob - the tumbeler probably bent the key. Thus you have a jagged piece of metal inside the lock. As said before, try WD40... and a pair of pliers. If it dosnt come out, you will have to replace the lock. You could also try turning the handle really hard the other way to bend the key back.
hmm. First off your suck its a door knob. or rather your sister sucks. IVe bent a key in a doorknob and it didnt get stuck. I reccomend the old wd-40 trick. if that dont work get someone stronger or take it apart