The answer depends on the device you are using and it's security setup. If you provide the make and model of the device someone here with knowledge of that device may be able to help. Alternately you can Google with the make and model for a device specific forum.
But my best guess is that as the wifi is unsecured, a security setting on your device has been changed to prohibit this type of connection. Either inadvertently by you of because of a system update. Look around the settings and try to find it.