Life as We Knew It by Susan Beth Pfeffer (2006)

Gripping post-apocalyptic tale of what happens when an asteroid hits the moon (long story short: nothing good). Narrated in a diary format by the teenage girl who survives with her family after the world falls after.  Followed by an interesting sequel called The Dead and the Gone, which follows a teenage boy living through the same disaster in New York City.  However, skip the final book of the trilogy (The World We Live In) which awkwardly ties both stories together with a convoluted romance.  Really badly done.