This sometimes happens when the main window for an application covers up a
dialog box. You can usually find the hidden dialog by pressing Alt+Tab to
cycle through open items.
If you're using Word as your email editor in Outlook then try deselecting
the "Use Word as your email editor" option and see if that clears it up.
If that's not it then please provide us more details on what you are
encountering and please include the application you are referring to and the
version.