Did you try to start it in neutral? You can have multiple problems. You need to start at the stater solenoid and see if you are getting 12 volts while the key is held in the start position. If you have 12 volts it has a bad solenoid or starter. General motors has issues with the starter solenoid sticking especially if it's hot. You can also hold it in the start position and whack the starter with a hammer. If it starts put in a new starter. If you don't have 12 volts to the solenoid check the neutral switch and verify that you have power going to the neutral switch while in the start position. If you have power going to the neutral switch but not coming out then it's the neutral switch. You also need to verify that the starting relay is good. Then you need to check the starter switch which is either part of the key tumbler or it can also be on the bottom of the steering column. You basically need to see if you have power coming from the switch, through the neutral switch, and the starter relay and to the starter solenoid. My best quess is the starter bushings are worn and when you try to start it especially while it hot. The armature will fall next to the windings and it can't create a magnetic field to turn the starter motor. So if you hold the key tumbler in the start position and someone taps the starter hard it will knock the armature off of the windings and the magnetic field will hold it in place and it will start. But most likely it won't happen again. I would have a mechanic diagnose the problem. If you take the starter into autozone or one of the parts stores and have them test the starter it will most likely test ok but won't work on the car while it's hot. It gets really hot in that area. I hope this helps