Ignition Switch suddenly won't turn !
 

I recently inherited a 1997 Mercury Grand Marquis LS from my father who passed away due to carbon-monoxide poisoning. It had 95,000 miles on it.

I drove it from Chicago to Phoenix - It ran perfect, cruised smoothly across the country. It got better gas mileage than the brand new V-6 SUV I just bought! I decided to keep it.

Then suddenly today the ignition switch won't turn. Won't budge at all. I tried different keys. The battery is not dead. I don't know the car very well yet. Is there something I did that I don't know about that caused this?

Does anybody have any ideas?