[Slade901]

Exactly, Year? Engine? Manual? Auto?

When you say the engine won't, do you mean that the starter is not even turning the engine?
or
do you mean that the starter is turning the engine but it just spins and spins then engine but the engine would not continue running on its own?

If the starter is not even turning over the engine, you may need to check the wiring to the starter and make sure they are tight and clean. Starters tends not to work if they are too hot which is caused by the heat generated near the exhaust manifold.

If the CTS (Coolant Temperature Sensor) is acting up and if the CTS is telling the engine that it is way too high (probably wrong temperature) and so it would not let the fuel injectors open properly.

Another possibility is the Safety/Neutral Switch is bad. Try starting the vehicle in Neutral.