My iPhone 3G won’t connect to any Wi-Fi and can’t find a network. I have tried removing the sim card and the problem is still unsolved.
What can I do?
iOS: Troubleshooting Wi-Fi networks and connections
iPhone 3G Cannot Find/Connect to WiFi Networks
Click on "Forget this network"
Turn off wifi on iphone
turn on wifi
reconnected to the network
try turning airplane mode on, and then enabling WiFi,
To be honest I'm not really sure what could be causing the problem beyond the fact that the device is quite old and may be on its last legs. I don't think a software issue is going to cause this - have you tried restarting your network equipment recently? This can sometimes be the reason (unless you've tried multiple networks).
Aside from that I'd say ensure you're running the latest software (if it's a 3GS I think that's iOS 5?) but ideally you probably want to start saving for a new phone.
If you wait a week Apple might announce the iPhone 5S, which will make the iPhone 4S and 5 cheaper and save you some money if you do intend on replacing it.