This is really bad news if you cant get the key to turn anymore. I'm sure you have been having this key sticky problem for a while and have just been jiggiling the key to get it to turn.

I have never done this before, but I have heard the only way to get the tumbler assembly out (which is what is bad in your case, NOT the ignition switch) is to drill it out.

Search the posts, there will probably be something on it.