Why Ignition switch does not turn to start engine?

If the key does not turn at all then its the wrong key.

wrong key as john states OR if your steering wheel is putting pressure on the steering lock this will also cause key to not turn try turning the steering wheel. try not to over power the steering lock so you don't damage it. try left if it wont move try right. if the wheel moves hold it and turn the key.. if this doesn't work then refer to the first answer. if the key has a chip then call the car dealer and tell them whats going on.. key may need to be replaced

wrong key, as stated, try jiggling the wheel back and forth (not forcefully) while turning the key. Or your ignition switch is going out

I have a 2000 Chevy Tahoe ignition switch will not turn over