Apple Mail Preview Window Disappeared... and fixed :)

My Mail preview window disappeared, and it looks like many people have/had the same problem, so posting what worked for me. After much research and trying various suggestions (clicking on the dot and trying to move it up and down, restarting Mail, restarting my Mac, deleted password protected PDF, etc), with no success...

I went under Mail, Preferences, Viewing, and unchecked "Use Classic Layout". I then clicked on "Mailboxes" so they disappeared and only a list of emails showed. I then clicked the right edge of the Mail window to get the double arrow and dragged it as far to the left as possible (making it smaller), and then without releasing, dragged it back to the right to expand until a preview window appeared to the right of the list of emails. I clicked on "Mailboxes" again so they displayed and then went back into Viewing and checked "Use Classic Layout" and it put the preview window back to below the email list where I wanted it.

I have no idea what caused the bottom preview window to disappear in the first place, but it happened when Mail crashed and closed on its own... from other posts it looks like it's not an uncommon problem... so I hope this helps :)