Preview Pane disappeared

I had turned my wireless mouse over to clean the bottom while still in Mail on m MAC Mini, and suddenly the window changed completely to a different format without the preview pane. It has disappeared completely and I can't find any means of restoring it. Please help me with the steps necessary to restore this feature, as I dislike the inconvenience hazard risk of having to open every email to see what's in it.

What version OS X, what version Mail yadda yadda....
Please read this, come back here and try again...

http://www.mac-forums.com/forums/news-community-announcements/313284-writing-quality-post.html

Thank you. My MAC Mini dates from about 2007. The OSX is version 10.4.11, and the version of Mail is 2.1.3 with (753.1) after it in parentheses. I had not downloaded anything, or upgraded anything, in a very long time, and it happened suddenly when I was trying to rub off accumulated dirt from the bottom of my wireless mouse. It would appear to me that in so doing I was pressing a pressure sensitive bar at the tip of the mouse, which is a Logitech Dark Field. Does this information help?
Thanks, 44gunfrigate.

This help you any? https://support.apple.com/es-es/HT2583