In Mail, on MacBook, I can't see main message on Mail screen

In Mail, on MacBook, I can't see a main message to show up on the main mail window . I see accounts, folder and mailboxes on the left and on the right a list of emails with 4 lines of the email. where did the far right part of the window go? I used to be able to click once on an email and it would show the email to the right. Now I have to double click the email and it opens in a separate window. This happened all of a sudden yesterday and I cannot find any view options to fix this. Do I have to eliminate a P list somewhere or something else? Please help me as I need this fixed ASAP.