You have a bad lock cylinder. This is pretty common on most GM cars. You can try tapping the end of the lock cylinder (where the key goes in) with the hanle end of a large screwdriver. Then insert the key and try to turn. Repeat this process several times. 99% of the time this will work. If you get it to turn than it needs to be replaced immediately. If you can not get it to turn than it will need to be drilled out to replace. The key needs to be in the run position in order for the locking tab to release.