The engine won't crank ?
First, check the battery and starter cables for their tightness or free of corrosion and physical damaged.
See this Starting system simplified diagram. Have a help from an assistant to crank the engine, use a 12 volt light tester or a digital voltmeter, apply it to the starter control circuit.
If there is light or 12 volts and the starter isn't cranking, then the starter is the culprit. Have the starter to bench test for free at the local auto parts stores.
If there isn't no light or no volts, then the ignition switch is the culprit.
Check the starter relay and position the Automatic transmission shifter in neutral.
Note: a 3/4 charge battery is 12.50 volts
A discharge battery is 11.75 volts.
A battery under a cranking load and it is reading less than 7 volts,it will be a recycled battery.