For one thing, 0% opacity corresponds to the window being invisible. 100% opacity is what you'd want for the window to be completely opaque. It sounds like you might have set your windows to 0% opacity thinking it was the opposite... and now when you log in they're all hidden since you told them to be. Then when you hit F10 it goes into 100% opacity mode (where everything is set to 100% opacity and you cannot change it).
I recommend going to Options, User Interface and hit the 'Reset Opacity' button. That will reset each window's opacity to default and make it so you can 'see' them when you log in.
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