This is the only explanation and cure that I have been able to find. Have a look at how to fix it if you have done similar. "a message had apparently been dragged down to about a one pixel size - then closed. Of course that means from then on messages were still one pixel in size when opened - unless I maximized them. They were there all along - I just couldn't see them"
This is the quoted solution:
1 Open a message (on the taskbar)
2 right click the task button, in the pop up menu click on "size."
3 Use the keyboard arrows to make the message big enough to see. I also had to use "move" in the same way to get it back on my screen.
4 From there I could drag the window out like normal, close it, and now messages open full size.