Check the key first, make sure its not bent, also try another key if you got one. Try spraying some penetrate/lube into the lock cylinder and if that doesn't work then use a small hammer and lightly tap the end of the key or cylinder as you try to turn it.
I cant remember exactly but you might be able to remove the lock cylinder and ignition switch as a unit and then it might be easier to try to seperate the two. If you can get the key turned to the "ON" position there is a small hole at the bottom of the plastic column shroud (under the cylinder) stick a small screwdriver in the hole and push the button up and remove the cylinder. With cylinder removed you should probably be able to figure out which one might be the problem.