One or two have had the switch fail in the "start" position - this will very quickly burn out the starter motor. The only option is to quickly disconnect the battery and stall the engine - which can only apply to manual models.
Chances are that one of the tumblers in the key barrel have stuck so try the graphite powder trick first (or even WD40 to loosen the sticky tumbler temporarily) - but if this doesn't work then it will be a major job to dismantle the ignition switch to fit a new barrel.
See the .pdf file with drawings which involves all the following:
Remove steering wheel / remove covers / remove transponder antenna / remove electric connectors / put ignition key in position APC / drill a hole 3 mm wide x 7-8 mm deep in shear bolt / remove shear bolt with chisel or stud remover tap / pull steering column all out and in lowest position / push down pin on top of ignition barrel with small screwdriver / pull out ignition barrel