Demolishing errors one update at a time.
http://www.nintendoworldreport.com/news/33759
Nintendo released a system update today, upgrading the 3DS to version 5.1.0-11U.
According to Nintendo’s website, this system update fixes an error some users received after the recent 5.0.0-11U update in March, which prevented access to the System Settings, Nintendo eShop, and Game Notes.
The update should completely fix the problem, but if you are still experiencing the error, related troubleshooting can be found here.