How to check the battery
A discharged or dead battery is one of the possible reasons for a car not to start. Sometimes we just forget to turn off the dome light or something else, or it could be some faulty component of the vehicle electrical system that drains the battery down. Sometimes, if the battery is old, it could just die one day, even if the day before it was OK. In either case, if the battery is low on charge, it won't have enough power to turn over the engine: you may hear some clicking noise or the starter may turn very slow when attempting to start the engine.
Here is a simple way to check if the battery is discharged:
Turn the ignition on. Without starting the engine, turn the windshield wipers on. If they move very slow, much slower than usual, the battery is probably low on charge.