ignition switch woes

[jos1@cornell.edu]

Has anyone here experienced sudden ignition switch failure?

Parked the car last night after running some errands;  today
when I wanted to start the engine, key only turned far enough
in the ignition to unlock the steering wheel -- no farther (sp?).

Tried the spare key also, with same result. So, how often does 
this tend to happen, is there an easy fix, and (if necessary) how
difficult is it to replace the tumbler assembly?

ANY input appreciated. OBTW: it's a 1989 100Q, and there was no
hint of impending failure.

jos1@cornell.edu