Any car battery will drain if the car hasn't been used for a while, so if you have been parked up for a while (especially over winter) it may be that your battery has been worn down. Things like the alarm will constantly use up power, so make sure you run your engine at least once a month or keep the battery on a trickle charger.
We always recommend using the car once every two weeks and running it for 15 minutes in a well-ventilated space.
You can also check out our battery health tips here.