If you are having trouble getting your key into an early 2000's GM vehicle, it is likely your ignition cylinder. It seems that this effect many GM makes, Pontiac, Buick, Saturn, Chevy, etc.
It seems the tumblers get out of line and block the key hole. Ude the nail file to gently pressing to align the tumblers and turn the ignition to the "OFF" or Locked position. You should be able to get your key in after.
Just be very careful when taking the key out until you can get a replacement part.

Video that helped me:    • Quick Fix: Getting your ignition key ...  

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