How can I change the order of messages in Mail for Lion? Suddenly the most recent message in Mail is at the bottom of the list instead of the top. How do I change the ordering of messages?

I am using Mac OS X 10.7.5, Lion.
Just above the rows of emails there is a dropdown that says "Sort by"...

This can sort by date, flags, mailbox, etc. 

At the bottom of this list, you can choose to have oldest or newest messages at the top. Check to make sure you have newest selected.