Is there a quick fix for a ignition key which will not turn on a 1999 sierra truck?

You can spray a graphite lock solution into the slot to see if it will free up. Don't use anything with oil, since that will just make a mess and when it dries it will cause the lock to start jamming up again. If that doesn't fix the probem, you may need to go to a mechanic, dealer or locksmith. Don't do it yourself since you'll need to remove the airbag to get to the keyswitch. BTW, make sure it's not just tension from the steering wheel. Work the wheel left and right while you're trying to turn the switch, see if that helps. Sometimes the wheel lock can keep the keyswitch from turning.