Well, unless someone can chime in with the answer, I'd try all the "Alt + <KEY>" combinations (do each of them twice at a time just to reset the condition of that particular window).
Or another thing you might try is to cycle through the F10 settings (which has three different visibility settings for the UI). The first (which is commonly used by default) takes all custom transparency/opacity settings into consideration. The second turns off all custom settings and show all windows that are normally visible at all times (certain ones like persona, knowledge book, casting bar/bubble, etc that can be either opened/closed on demand or only show when certain requirements are met won't unless you do what you need to do to make them appear). The third one disables the UI altogether.
Try toggling the second F10 display setting and cast a spell to see if the window appears. If it does, then you must've somehow tweaked its transparency/opacity settings (via right clicking on the window and selecting Window Options).
If neither of those routes work, the window may have been moved off screen somehow. You can either go into the UI section of the games options and select/deslect the option which controls the windows' ability to be moved off screen or you can back up your UI settings file (<SERVER>_<CHARACTER>_eq2_uisettings.ini found in your base EQ2 directory) and create a new one which should also remedy your issue in the long run.
"Certainty of death! Small chance of success! What are we waiting for?!"