How do you bring back lost window if CTRL +Shift + T or CTRL+Z don't work?

What software have you recently installed since it stopped working?
Some application is not allowing that to run as expected.
Restarted the computer and disable one by one all the background applications. To do this go to Apps and Features Advanced Options.

@Forever_Free Thank you. I’ll keep that for next time, but for now I’m looking for something simpler. I haven’t installed anything.
I haven been on Fluther till just now.

@chefl Sometimes there isn’t a simpler. Some application has stollen that keyboard shortcut. Good luck finding it. Its a pain to disable background use. So many application want to run in background that really don’t need to.
I assumed you were running Windows OS. You can also go to you Hidden Icons in the lower Right near the time and click on the ^
The hover over each icon and right click to quit these applications one by one to see if you get the shortcut back.

@Forever_Free it is Windows 10 by the way. As an aside, 1)why does an application want to steal a short cut? Is it a virus? 2)And what can be done as preventative measure?

1) no. it is by design. Some applications register those keystrokes to a function of theirs. Then it becomes a battle.
2) Nothing. Just awareness that some application register default shortcut keystrokes.

An example is an application i use a lot is Snagit. It allows you to screen capture and other functions for creating documentation and showing others examples. They have default shortcuts, but can go into settings and change them and add more.

@Forever_Free I’ll never undestand it. Thanks for trying. So, it’s on any kind of device then?

it is part of the installation and configuration on the OS.