Servers for the demo of Metroid Prime: Federation Force Blast Ball will be shutting down forever by the end of the month. If you want to keep playing, you will need to buy the full game.
Nintendo announced on the eShop: “Thank you for your continued support. The online multiplayer feature for the Metroid Prime: Federation Force Blast Ball demo will be discontinued at 12/31/2016 23:59, PST. The discontinuation of this demo feature will not impact the online multiplayer feature available in the full version of the game.”
This is common practice in the video game industry as servers cannot be supported forever. Nintendo also recently shut down the servers for Devil’s Third on Wii U too.
If you don’t want to buy the full game, you have until tomorrow to play the demo as much as you want before the servers are dead forever.