When your battery suddenly starts going dead it could be that the battery has developed bad cells inside the battery or you could have something such as a bad alternator shorting the battery out as well.
Also check to make sure you didn't leave a cell phone or any other 12 Volt items plugged into your cars 12 volt system as some cars will not shut off the 12 volt plug when you turn your car off and that can drain a battery.
I know that from personal experience. I left my tablet in the car charging overnight because I forgot to bring it in the house and my battery was drained overnight.
It could also be something else shorting the battery out or you have dirty or loose battery cable connections not allowing the battery to fully charge back up when you're driving.