When you connect to a password-protected Wi-Fi network using an iOS device - iPad, iPhone, or iPod touch - your gadget stores that password for use the next time you connect. Makes sense.
But what if the WEP, WPA, or WPA2 password for the Wi-Fi network in question is changed? Your device may try to connect and then tell you it can't, or in the case of an iPhone or 3G-capable iPad, just silently drop back to 3G data service, with the only indication being the 3G status icon appearing at the top of the screen when the Wi-Fi status icon should be there.
The solution is enforced amnesia; I only recently figured out that this would resolve some problems I had suffered from in the past.
To work around to the now-incorrect password being stored, make sure you're in range of the network that has had its password changed and follow these steps:
In the Settings app, tap General > Network > Wi-Fi.
Tap the blue details arrow to the right of the network name in the Wi-Fi Networks list.
If the password is stored, you'll see a Forget This Network button at the top.
Tap it, and confirm the action.
Return to the Wi-Fi Networks list, and tap the network name (not the details arrow).
iOS prompts you for the network password, which you can now enter to reconnect to the network.
Useful as this trick is, Apple should update iOS to prompt the user when a stored password for a remembered network no longer works.