Occasionally when you add a new monitor, or shift them around, the Save As dialog (or other modal dialogs like Find & Replace) gets lost outside of the viewing port. It’s usually sitting just above your smallest monitor or something similar.
To fix the situation in Windows, you can open up the Screen Resolution applet from the Control Panel and move a monitor up relative to the others then click Apply. You should then be able to it then.
Another thing you can do is to switch the primary monitor from one to another. Check the “Make this my main display” checkbox then click Apply. You can reverse it later.
It’s an annoyance for certain. However, the root cause is a “damned if you do and damned if you don’t” situation.
Windows allows you as a developer to put things off the screen. There are valid reasons for this, but when it interferes with normal computing, it’s a major pain, and one that most people simply can’t solve.
Still, the cases where you’d want to do it for a modal dialog are pretty bizarre.
What prompted this? Adobe Photoshop. 😉