Problem with mail

Hi i am new to forum, am sure this will be simple for everyone, but i cannot figure out. i have leopard osx, when i open my mail previously in any mailbox, the window was divided into upper & lower parts with the subject of mail in upper part & the message in the lower part. This is what i want, but i inadvertantly did something & now i just get the subject & have to click on it to view & everything is in one section or window. i cannot figure how to set it back. i hope that this is clear enough & that someone can help.
Thanks very much.

I think I understand what happened. At the bottom of your Mail window there's a thin bar. Click and drag it up.