Game Files Missing.

I go onto the Minecraft Launcher like any other day to be greeted with a newer looking launcher. This is fine. I click "Play Demo" and am greeted to, "Some game files are missing, and can't be downloaded while the launcher is in offline mode. Please connect to the internet and try again." I don't remember affecting the Minecraft files, so I'm confused as to why I get this error. I tried deleting and re-downloading the launcher. Nothing.