I had a report that the AutoCAD® Architecture Open dialog was not showing up when the OPEN commmand was being used. First thing I checked was the value of the FILEDIA System Variable, but it was set to 1, and there were no prompts on the Command Line when running the OPEN command.
I found the solution for this case in an Autodesk Knowledge Network article, Dialog windows do not display and the program appears to freeze, which offers several suggestions for resolving the issue. Having already done the first one (FILEDIA), I tried the second one - holding SHIFT+WINDOWS KEY and then pressing the LEFT or RIGHT ARROW KEY. That keystroke combination moves the current window from one screen to the next, and it did bring the Open dialog back onto one of the two active monitors.
The user's computer is a laptop, which was docked and closed, but perhaps the dialog was hiding out on the laptop monitor? Wherever it was, the issue was resolved, and I was glad to not have to resort to the last suggestion, editing the Windows Registry.