found the solution - in trying to alter the window frame size, I set it i "0" which might have removed the glass. After resetting to default size, the opaque windows reappeared. Thanks all for your suggestions.
yep - I'm working with ancient version 10. XP operating system.... only one of my many saved plans has the transparent windows. I have tried 3D>3D settings>options>vector view options> check opaque glass from outside.