Hello stephan51,
Thank you for using Apple Support Communities!
I would recommend starting with this article named iOS: Troubleshooting Wi-Fi networks and connections found http://support.apple.com/kb/TS1398.
First the general troubleshooting:
- Be sure that you are in range of your Wi-Fi router (access point).
- Tap Settings > Wi-Fi and turn Wi-Fi off and then on again.
If unable to do so, see this article.
- Confirm that your Wi-Fi router and Cable or DSL modem are connected to power, turned on, and indicate that they are connected to the Internet.
If not, refer to your network administrator or ISP for assistance.
- Restart your iOS device.
- Tap Settings > Wi-Fi and locate the Wi-Fi network to which you are connected. Then tap > and Forget this Network. Try to connect to your desired Wi-Fi network again.
Note: You will need to enter your Wi-Fi password again if your desired network requires one.
- Turn your Wi-Fi router off and then on again2. If your ISP also provides cable or phone service, check with them before attempting this step to avoid interruption of service.
- Update your device to the latest version of software.
- Update your Wi-Fi router to the latest firmware2.
For AirPort Base Stations, see this article for information about firmware updates.
And then the more advanced troubleshooting toward the bottom if needed. There are different instructions depending on the symptoms you are seeing.
Cheers,
Sterling