Check for a low car battery?

Try starting the car. If it doesn't start, then you might have a low charge in your battery!

Turn the headlights on and then try to start the car

- if the lights dim and you can hear the starter motor solenoid "click" or the starter turns the engine slowly then your battery is more than likely At Fault.

-if the lights don't dim and you can't hear anything from the starter motor then it is likely to be a loose earth connection or a loose connection to the starter motor solenoid (the thin cable out of the two that connect to your starter motor)

Simples.