When you open a dialog box the dialog box will disappear. The problem is the dialog box is still opened somewhere and is waiting for input.
If you Right Mouse Button (RMB) over the desktop you can select "nView Properties", which brings up a dialog box for "nView Desktop Manager".
Select the "Windows" tab.
Prevent windows from opening off-screen": ENABLE (checked)
Dialog box repositioning:
Reposition dialog boxes on: Move to application display
Center dialog box on display: ENABLE (checked)
Also, please delete all files with content *wnd* (Wnd=Window)., when it happens, in
For Power ProStructures (V8i)Drivename:\Program Files\Bentley\ProStructures\V8i_S6\AutoCAD 201x\Localised\<Country setting you used>\Temp\Metric
For Prostructures on Microstation its in
Drivename of installation:\ProgramData\Bentley\ProStructures V8i S8\WorkSpace\ProStructures\Localised\<Country setting you used>\Temp\Metric
For Prostructures CONNECT Edition its in
Drivename of installation:\Users\<User name>\AppData\Local\Bentley\ProStructures\10.0.0\prefs\Personal_WndCfg
That sets all dialog to default position, as those files store the dialog positions.
It´s mostly needed to delete these files when you switch monitors or resolution.
Or use some extra downloadable Tools, there is a tool include that resets all dialogs to default position.
WIKI: Reset Dialog Positions
Download link for ProSteelTool: ProSteelTools SelectSeries 6 for Autocad Version 12.11.2013