I have multiple keyboard layouts installed for the US English language. In the past, I was able to hit Win+Space to switch between them, but for some reason that is no longer possible. I have no idea what has caused this so stop working, I didn't change anything from a week or so ago when it was working fine.
According to this complete list of Windows 8 global shortcuts, Win-space should still switch keyboard layouts.
This would indicate that some application you've installed has stolen the shortcut. It appears to be very hard to find out what application has taken a global shortcut but there are a couple of tools that claim to do it at least in part. Note that I've not tried these myself and I don't know if they work under Windows 8:
Let us know if they work.