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.
There are two main reasons for a car not starting. One is lack of spark, the other is lack of fuel. Lack of spark can be caused by a dead battery, a dead starter, a faulty ignition or bad spark plugs. A lack of fuel is usually the fuel pump or problems with the EFI system. A dead battery or alternator is fairly easy to diagnose because the rest of the electronic systems will either be dead or dim. A clicking sound means the starter motor is trying to engage, but failing. No clicking sound means that it is either the ignition or the starter solenoid.