This fix requires modifying a file that Revit uses to build its UI when it is loaded. First, make sure Revit is closed. Navigate to the Start menu and then to Computer. Pressing the Alt key one time will bring up a hidden menu. From this menu select Tools then Folder options.
A new window will come up with 3 tabs. Navigate to the View tab. In the middle of the Advanced settings section there is an option that says Show hidden files, folders, and drives. Make sure that option is selected and click Apply then OK
Now, double click Local Disk (C:), Users, then your login name for this computer (mine is Assemble Systems), AppData, Roaming, Autodesk, Revit, your version of Revit (mine is Autodesk Revit Architecture 2013). Your window should look similar to this.
There is a file called UIState.dat. This file needs to be renamed to UIState.dat.old. When you try to rename the file a message will come up asking if you are sure you want to change the name. Click yes. Close out of this window and check to see where the Assemble tab is.