I recently ran into a strange pgAdmin4 bug where the UI loaded but none of the menus or dialogs were visible. This is apparently the result of a bug which can corrupt the configuration settings. Fortunately the fix is pretty easy. Unfortunately, you will need to reconfigure your UI customizations.
Close pgAdmin4 and then browse to the AppData folder. It will depend on your installation, but for me it was located here:
Delete or rename this folder and your pgAdmin4 is miraculously fixed!