A little more detail on what's happening when you try to start it will help. Any lights or noises? The car being locked or not doesn't seem like a likely cause, more of a coincidence.
How long have you had the car? If you bought it used, I would think the battery is a possibility. But your description of what's happening will help everyone here help you.
Answers
How long have you had the car? If you bought it used, I would think the battery is a possibility. But your description of what's happening will help everyone here help you.