Rx8 won't start or do ANYTHING

Hey guys, so I was going to go out today and when I tried to unlock my car, it wouldn't unlock. So I had to use the actual key to get in. Then, when I tried to start it, NOTHING happened. No lights, no cranking, anything. It was just, dead. I got a new engine about a week ago. I don't know what it could be. It was raining yesterday and my car was outside. Someone help </3



Edit: I didn't even think about the battery. Could that be it?