2003 Century w/ dead battery

So I left the lights on in my car, the battery died. The car was locked. Now it won't open with either the remote or the manual key. I can't get the hood open to try to change the battery. This all seems too simple, but I still can't figure it out. What is up?

Thanks

Welcome to AF.

I don't see why you are having problems unlocking the doors manually. Buicks and other GM cars come with a spare key to unlock the drivers or passenger doors in the event of malfunction. It is the key without the chip. It should unlock the trunk too.