Is the gear column in reverse ? also normally it is the ignition switch its self that get jammed up and stops the key from turning, so there are two screws on the backside of column (Phillips) under the ignition where the key goes, you have crawl under there and undo the screws, then the switch can be pulled straight out, and then the key should freely turn. and you probably just nee a new switch.
Could also be the reverse lock out, but start with the switch first.