I try using Alt-Tab to switch to another window, and could see the window in Alt-Tab task switcher, but when I selected it, it didn’t open. This happened to me, for example, with the Text Editor and also the File Manager, and happened multiple times before.

It turns out you can send windows to another virtual workspace by mistake, even if you don’t have workspaces enabled, which they aren’t by default.

I solved this and found my missing windows by:

  1. enabling workspaces, and then
  2. switching to the next workspace, where I found my missing windows and then sending them back to my primary workspace.

1) To enable workspaces, go to: System Settings > Appearance > Behavior and click on the Enable workspaces checkbox.

2) To switch to the next workspace, use the keyboard shortcut: CTRL+ALT+RightArrow

Then select the window you’ve been missing, and send it back to the first workspace if you’d like, with: SHIFT+CTRL+ALT+LeftArrow