When attempting to open a model from File > Open, the dialog to locate and open the file does not appear.
First, check to make sure the window did not appear on another monitor, and confirm that the Windows File Open dialog appears in other applications. Try rebooting your computer if other applications are not working.
In at least one case, a "clean" reinstallation of the OpenFlows product resolved this issue. See: How do I perform a clean uninstall and reinstall of the Hydraulic and Hydrology products?