It is like I said before. Normally you have to select it in the menu View and when it is not reappearing on the same place like before you have to dock it there. If it is already selected it is somewhere hidden on your screen(s?). Look
A "tip" from the online Help, Index, window docking area
"When the Allow floating windows to dock check box on the Display tab of the Preferences dialog is cleared, windows will not dock unless you hold the Ctrl key. When the check box is selected, you can prevent a window from docking by holding the Ctrl key."
The option there is to "Allow floating windows to dock (hold Ctrl to override)".