Nintendo didn't explain the cause of the downtime, but advised that the service was back up.
Nintendo on Twitter.
Nintendo's follow up tweet.
You can check the Nintendo Operational Status page for future occurrences.
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Nintendo on Twitter.Unfortunately, we’re having some issues with our network services. We’re looking to rectify the situation as soon as possible. Thank you for your patience.
Nintendo's follow up tweet.The issue with our network services has now been fixed. Thank you for your patience!