in Mail, my preview of incoming emails changed to one line only. How can I get the bigger view?

My view of emails has changed suddenly to only one line. It used to be a bigger window, allowing me to read part of the body of the email, while viewing all new emails....how can I get that view back? Is that a long header? I try to click on long header but it is greyed out and unavailable even though I added the long header icon to my task bar. Help??