Nintendo has scheduled some routine network maintenance for tomorrow (September 11th). This maintenance is only for the Nintendo Network ID Login which affects Wii U and 3DS systems , along with Nintendo’s web services on PC and mobile browsers (like the Nintendo Game Store, for instance).
The work period will last for several hours: September 11th: @4PM – 8PM (EST). That means you won’t be able to use any network services on Wii U and/or 3DS systems requiring your NNID during the maintenance period. This may be an inconvenience for some, but at least it won’t last an excessive amount of time.