[ShadowProwler420]

Sounds like a few possible things have happened.

a) The window has been moved off screen -- If this is the culprit, there should be an option to check that keeps all the windows from going off screen (and shifts any windows currently off screen into view)

b) The transparency/opacity settings have been maxed out on the window --If this is the culprit, hit F10 to shift the UI into a 'full view' mode (any custom transparency/opacity settings are disabled) and reset the settings on the window

c) The file for the window has been corrupted somehow -- If this is the culprit, delete the file and re-download it (or, if you don't know the name of the file (I don't know it offhand, either), it may be easier to just delete your extremeui directory and pull down a fresh set of files altogether).

d) You uisettings/window placement file (<character>_<server>_eq2_uisettings.ini) might have become corrupted -- Delete it and either let the game recreate it (which will force you to manually arrange the windows), or (if Landiin provides them) get a new one from the extremeui autoupdater.