What could be wrong, why won't?

Dead battery ? If nothing else works, ie radio or head lights thats probably the case. Or if this is some kind of riddle maybe you should turn the key once its in the ignition. If the lights radio accessories etc work then its probably your starter.

Try the battery leads see if they are corroded or even too dry take them off, scrape them clean and put a smear of lubricating oil on them, this prevents further corrosion. if there is a lot of corrosion wash off with clean water.

If you have a battery charger disconnect the leads before using it..

You could need a new battery if yours is something like 5 yrs old. after that age they can die overnight.

Maybe the starter went bad?

no sound no nothing?

could be a dead battery.