My pag has only the scrolling e-mails-I have lost the content of the highlighted e-mail on the page-pls help!?

When I click on a mail message at the moment-I now need to double click it and I lose my scroll of e-mails. Before I had half of my page with the scroll of e-mails-then half the page with the e-mail that I highlighted being below to read...what have I pressed to lose this?! Hope this makes sense!