Finder preview automatically goes full screen whenever Finder gets focus
Using the answer space to provide images, but this is a guess...

I'm assuming you've rebooted &amp; this hasn't fixed it?

I've never seen fullscreen Quicklook get stuck like this before, but I have seen regular Quicklook get stuck, usually if Quicklooking an attachment in Mail, then forgetting &amp; switching to another app/Space.
The fix is usually to get back to Mail, reselect the original item &amp; click the close X top left; as none of the regular methods will work at that point.
All I can imagine is you've got stuck in a similar situation, but in fullscreen Quicklook.

Ordinarily, you would click the double arrows to get into fullscreen...



Which would give you this...



Note the floating section at the bottom...



This will vanish if you don't move the mouse, then reappear if you do. Esc will also revert to windowed.

On a complete guess, I think Spaces is what's trapped you; my proposed solution may mess up your usual Spaces structure which you'll have to rebuild afterwards, so it depends on how much you use them as to whether you'd try this as a first off...

Quit all apps using fullscreen or spread over more than one Space.
I'd be tempted to have only a single windowed app running, in Space 1, with only one window open.
Shut down, unplug all except your primary screen.
Boot.
This will force all Spaces to that monitor &amp; I think will then let you out, simply using Esc, or the floating controls.