Clutching at straws but did you check that any and all settings that were relative to a 2nd monitor were disabled?
There are a few options out there if you search online for "move application that is off screen with keyboard".
The one I've used is to hold shift and right click the application in the task bar then select Move. Then you can use the arrow keys or your mouse to bring it back into view.
To find lost windows (not just SW ones), I wrote a utility, posted here: Find lost dialog/window utility (using Autohotkey) V1.01 update
You'll need to enter a case-sensitive portion of the title.
This is the SW title, it's not just the currently active file's path and file name:
So enter SOLID (this should be unique enough) in all uppercase and hopefully the utility will find SW and move it to your main screen.
EDIT: That's obviously an English install title tooltip shown.
To get your local language install image of the same thing, let your cursor hover over the SolidWorks tab in the Windows taskbar.
Eventually, a tooltip box similar to mine should display (Windows 10, SW2018).
You don't need to have an SW file open for this to work.
This is what's displayed with just SW running:
The text you enter into the utility's input box must uniquely match what's shown for you, including case.