https://www.techspot.com/guides/226-bring-lost-window-back-desktop/
How to Bring a Lost Window Back to your Desktop
Windows 10
From the taskbar select the application you're having problems with. The program's window will be "active" but out of view for now.
Leverage Windows 10's Snap Assist capabilities by using one of these two keyboard shortcuts:
Snap a window left / right: Windows key + Arrow key left / right
Snap a window to a quadrant: Windows key + Up or Down (after moving left or right)
If that doesn't work. Use the screen resolution trick:
Right-click on the desktop, choose "Display settings." Scroll down to the resolution setting, and change your display's resolution to a lower value temporarily. That will force a window rearrange and the lost window should be back on display. Before changing the resolution back to its previous setting, manipulate the window (maximize or resize), then go back to normal.
Windows 7/8
Bring the troubled window to focus by clicking on it in the taskbar (or Alt+Tab). Now you can simply hold the Windows key on your keyboard and tap the arrow keys. With any luck, your missing window will snap back into view.
If that doesn't work, you can use another keyboard trick: Select the application window by clicking on it in the taskbar, then tap Alt + Space, which will open a menu on the missing window. Obviously you won't be able to see the menu, but can still interact with it. Tap "M" on your keyboard to select "Move" on the menu, and then use the keyboard arrow keys to reposition the window back to your desktop.
If all else fails, right-click on your taskbar and select "Cascade windows".