I found a solution to the issue. <kbd>Opt</kbd><kbd>Cmd</kbd><kbd>Space</kbd> has always been a command for showing a "finder search window". If you now just see a window without the sidebar just do this:

 - press <kbd>Opt</kbd><kbd>Cmd</kbd><kbd>Space</kbd> To open this window
 - Press kbd>Opt</kbd><kbd>Cmd</kbd><kbd>T</kbd> or go to "view>hide
   toolbar".

Sometimes we just press it accidentally and this hides the toolbar, which makes this searching window into a "normal finder window".