This happened in my Chev truck. The ignition would not turn. The remedy was to use tri-flow in the lock to free up the tumblers, using the key on a small set of vice grips and wiggling and turning for about an hour. Apparently, the tumblers had accumulated some dust over time an caused the pins to get stuck.

Eventually, the lock freed up and was able to turn. I used a liberal amount of tri-flow in the tumblers until when wiped off with a cloth it looked free of debris.