Yosemite operating system - mail
Please try this.  Click on Mail > Preferences > Viewing  There will be a dropdown for list preview.  Select a choice other than None.  That will give summary information about the email message you have selected. Make sure you select an email. The actual email message should appear to the right of the summary.  If it does not, you'll probably need to move your cursor as far to the right of the screen as you can go.  Push down your mouse or track pad and move the cursor to the left.  You should see the email that is selected.