I clicked 'Forget this Network' on the Wi-Fi network, and I need to use that network?

I'm on iPhone 4, with iOS 6, and like I mentioned, it was an accident and I need to use that network!

Just set the network up again as a new network on the phone. You must have been through the set up procedure before, and now you have "forgotten" it you need to repeat the process.

Search network again and it will appears SSID which you were using and connect it. Give key. Hope this wil work.

So use the entwork icon to look at available networks, Select the one you want and when it asks for the security code enter the correct code for it.