The battery is dead, so I cannot start my engine at all.
The battery is a key component for the car to start. The car uses the battery at start up to provide power to the starter. Once the car starts the alternator takes over the job of supplying the car with the electricity needed to to run. If the alternator fails the car simply takes the electricity it needs to run from the battery until it drains it. So a dead battery can just be a dead battery because it simply wont hold a charge anymore, or it can be dead because something drained it. A mechanic should be able to test your battery to see if it needs to be replaced.
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