I don't know what made it stopped working. I use BetterTouchTools, but not command has that shortcut and even when I close it, Command + Space still doesn't work.
I checked System Preferences → Keyboard → Shortcuts, and there is no yellow triangle warning, and again Command + Space has no other conflicts. I've restarted my computer and still the problem persists.
I don't know what else to try. Is there any way to check what is being triggered when I press Command + Space key sequence?
I am using a 15" 2017 MacBook Pro running macOS High Sierra 10.13.6.
Edit: This was on a work computer I no longer have access to, so I can't try any solution to see if they work.