Mail won't view as HTML

While I was reading my emails, my Mail window changed on me. I lost my view window… I used to have three columns, now I only have two. I have the mailboxes column on the left, then the column containing the mail for whichever inbox or folder I want to view, but the right hand column which displayed the email when selected just vanished from the entire Mail window. Right now the emails display only the first one or two of the email text and I must double-click an email to open it, which opens it in a completely separate window where it then displays the full contents including any HTML if it has any… it's as if my viewing window came detached from the entire mail window, and I can't figure out how to put it back. I can no longer scroll down my emails and just select one to see it's contents displayed in the right column anymore. How do I get it back? It's very annoying and wastes time to have to double-click on each and every email to open it.

I'm on Mountain Lion on my 2011 MBPro and am using Mail 6.6.

Thanks in advance!