Same as an auto trans, there is a cable that attaches to the reverse shift lever on the trans. That cable runs up to a bracket on the steering column, below the brake booster. Your cable may be binding, or kinked, or have some other reason it is not allowing the shift lever to move the way it should. You can disconnect the cable from under the hood for now, if ya want, and then manually move the lever it is attached to on the column UP, and tie it there. That way, you can at least get the key out. The cable is held on by a small clip. Just remove the clip, slide the end of the cable off of the lock lever, and there ya go.
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 Not to worry, easy fix. Under the dash, inside the car, beside the steering column is a bracket with elongated holes. Loosen the two bolts that hold it, rotate the bracket slightly eather way, which moves the cable end, which alignes the tab to allow the tab to coincide with the key way. Move the bracket enough so when you turn the key it will engage and allow release. OR just move the shifter in and out of park a couple times, it may allow you to remove the key. In the end it may need the adjustment I outlined above.  Sounds like a long ordeal, but really quite simple.