Option + Command + Spacebar opens Searching this mac window instead of Finder
I found a solution to the issue. OptCmdSpace 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 OptCmdSpace To open this window
Press OptCmdT 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".