How do I un ignore a network on iPhone 4s

When in range of the network, just re-select it & connect.

Forgetting a network does not block your ability to re-join the network, or from the network appearing on the list of available networks. All it does is stop joining automatically & erase the password. So, if your phone does not see the network, either you are not in range; the SSID is hidden; the network isn't broadcasting; or something else is going on.