1998 GMC Jimmy ignition

Look under the column on the plastic cover. You should see a small plastic plug in a hole. Remove that plug and stick a pen/screwdriver in there. You will find a plunger that you can push up. If you push it up and the key moves freely and you can rotate it again. You can remove the plastic cover around the column, and go in and remove the solenoid that plunger is part of. It is the interlock solenoid which is intended to prevent you from turning the key to the lock position if the vehicle is in gear and in motion. I've had a couple of them fail and act this way.

If you push up the plunger and the key doesn't move freely, you will be replacing the ignition switch tumbler assembly. It isn't hard to do but does require a couple of tools (available for rental at some parts stores)to do the job.