What do you do if your 93 Pontiac Bonneville wont start after changing the ignition?

Changed the ignition?? The key tumbler? The switch itself? The parts under the hood? IF the key, was the part in the column in good condition. Did the key assembly seat properly? If the switch itself, it is adjustable to a degree and may not be in the right place. Under the hood? Depends of what was changed.