I have a book idea, but not sure if it's good

I have an idea for a book.  Please let me know if anyone thinks it's good:

A girl wakes up one morning and there is complete mayhem.  Everyone is screaming and running from something.  As the girl is running, she falls and hits her head. When she wakes up, everyone is gone, destruction is everywhere, and she doesn't remember what happened.  She doesn't know what happened to the human population, but as she is searching, she finds a boy also looking for everyone.  At first she doesn't like him because he treats her as if she's defenseless or something like that. But as time goes on, she falls in love with him because of certain events.  I was thinking about making the thing that attacks something along the line of "WWIII" with the terrorists/enemy coming and destroying everything.  I was also thinking that if i run with this idea, I could tell of how the protagonists are captured by the enemy and how they escape.....

let me know what you think!

Thanks!