Mac Mail viewer window doesn't open
I am running Maverick on a 2015 macbook Pro with retina display.
When i startup mail, the application starts up however a window does NOT open. Only the drop down menus at the top of the screen are visible. This is new. When I have started mac mail in the past the viewer window opens.
MacBook Pro (Retina, 13-inch, Late 2012), OS X Mavericks (10.9.5)