Are you using a smaller resolution than you used to ? the window may be being drawn off of the visible screen -- try deleting your Constructionset.ini and let the CS create a new on with default values and it should come back ! (the ConstructionSet.ini is located in your My documents\My Games\Oblivion\ folder and if you'd like instesd of deleting it and returning everything to default just open it in a text editor (ie. Notepad) and look for these lines :
Render Window X=232
Render Window Y=179
Render Window W=956
Render Window H=871
they are the ones you'll need to adjust to get it back and represent the number of pixels in and down from the left top of the screen and the width and height the window is drawn also in pixels -- so make sure they are on your visible desktop space and save the changes !