Why won't my buick regal start?

Gear selector switch sending the wrong signal, ignition switch not sending the crank signal when placed in crank position. Bad wiring.

In a twenty-three year old car, it could be just about anything. Only someone with a mechanics skills can truly find the cause. As the common denominator is juice not getting to the starter solenoid, I'd suspect it's the Ignition switch is broken, or the gear selector is out of adjustment; try wiggling it in neutral next time. It could also be a broken battery cable. Any of these is easy to diagnose.

Anti theft can cause that. Have it checked for codes.

Does your ignition key have a visible chip on it? If so you have the VATS (Vehicle Anti Theft System) security system. Assuming you tried everything that's been suggested, the most likely culprit is the ignition lock cylinder. There's two tiny wires inside the ignition lock cylinder that must contact the chip on the ignition key. They tend to break. You can try cleaning the ignition key, especially that chip, since it might just be dirty and not making good contact with the wires when you insert the key. Does your "Passkey Fault" or "Security System" light come on when you try to start it? If either or both lights come on and stay on, it's a security system problem. Here's where it gets tricky. First you need to measure the resistance of the chip on the ignition key using an ohm meter. That will tell you which VATS key you have. There were 15 possibilities, more on that in a moment. Next, remove the lower steering column cover. Look for two wires, one black and one white inside of an orange sleeve. Unplug them. Now insert the ignition key into the ignition lock cylinder and measure the resistance across those two unplugged wires while you turn the lock cylinder through all the positions. Wiggle and jiggle the key as well. If you get no resistance reading or the reading is erratic, the lock cylinder is bad. Now, the new lock cylinder will come with a key template that WILL NOT start the engine! You need to use the resistance reading you got from the chip on the original ignition key to determine which VATS key, 1 thru 15, you have. Then you need to buy the correct VATS key blank. Then use the template key that came with the new lock cylinder to have the VATS key blank cut so it will work with the new lock cylinder. However, if you find the ignition lock cylinder is good, you need to check the BCM (Body Control Module) for trouble codes and go from there. Several years ago I went down this road with my '94 Fleetwood and wound up learning how the system works and how to fix it. If you get no codes from the BCM, the ignition lock cylinder is good, and it's definitely a security system problem, replace the BCM. If you're going to have someone else fix this security system problem show him these instructions. Without them I can guarantee he'll have absolutely no idea of what he's looking at or how to fix it!! Jim