How do I get the iCloud mail message bodies pane back?

When in my apple iCloud email, I dragged the message list box all the way to the right. Then the body of my email's disappeared completely. Even when I drag the size of message list back to the left, I have to message body at all. It was due to my applying some command. How do I get the mail message bodies pane back.

[Re-Titled by Moderator]