This can be caused by using Unity on a PC that alternates between single and dual monitor layouts. If a window is left on a second monitor one day, it may not show up when in a single monitor environment the next day. Sometimes adding a second monitor and dragging the window back onto the host monitor will resolve this. If all else fails, you can alter the registry editor to reset everything to the default settings.
****Please be advised that by going into the registry editor you can render the computer unusable by altering the wrong items. If you are unfamiliar with or unsure about using the registry editor, please contact your IT department. On some PCs, you will not have access to the registry editor at all.****
Close Unity Pro.
Using classic windows, Select: Start - Run then type in regedit and hit OK.
Select: HKEY_CURRENT_USER / Software / Schneider Electric / Unity Pro XL / ProductVersion0 and delete Options key
If this does not fix it, you will need to reinstall.
Legacy KB System (APS) Data: RESL212292 V1.0, Originally authored by DaSo on 08/12/2013, Last Edited by DaSo on 08/12/2013
Related ranges: Unity Pro - SoCollaborative software