This guide will help you if your Wi-Fi settings have been deleted.
This usually happens when a Wi-Fi compatible game has been played in a different Nintendo DS, as the Wi-Fi settings and stored data are specific to each system.
Your Wi-Fi settings can be configured at any time, and individual games will save their data on their Game Cards.
To reconnect your system to the internet, follow the advice from How do I connect my Nintendo DS to the internet?
Your Nintendo DS system’s manual will have more information. You can also contact Nintendo Customer Support here, or call +44 (0)845 605 0247.