Apple email message panel will not open. Sierra 10.12.6 MacBook Pro

I have a  MacBook Pro Running Serra 10.12.6.  My Email message panel will not open.
Howie KaySystems Engineer Lockheed-RetiredAsked:
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David AndersTechnician Commented:
Mail opens? Preferences under Mail menu opens? View Menu drops down? Window Menu drops down?

Window Menu > Message Viewer [Command+O] will not open message viewer window?

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Howie KaySystems Engineer Lockheed-RetiredAuthor Commented:
Hi Dave,
Thanks for your help.
Everything works as expected in my Apple Mail app with the exception of the message panel.   The panel cannot be expanded by trying to raise it from the bottom of the blank panel with the mouse cursor showing an up arrow.  In non classic view I am able to display up to 5 lines in the group email listing panel.
Howie KaySystems Engineer Lockheed-RetiredAuthor Commented:
Sierra repaired the problem after several weeks.  It was fixed.
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