My guess is also that it was undocked and moved offscreen.
Close max, search for a file named "docking.cfg" in some SME subdirectory, backup it temporarily and delete it at it´s original position, then restart Max.
The SME directory is either in your user folder (AppData/Local/Autodesk/3dsmax/(version)/enu/sme or such) or directly in the max application directory.
Good Luck!