Peugeot 206SW Horn sounded until battery disconected?

Pressed deactivate alarm on key. Horn sounded continuously. Could start car, lock/ulock it with activation on key etc, no problem but Horn would not stop until battery disconnected. Then replaced terminal on battery and horn not sounding. Have only deadlocked car for now, but has anyone any idea as to what the problem could be? Obviously now I have no radio as code not supplied by dealer when I purchased the car! So if anyone has any help on that too I would be grateful. (Widow relying on neighbours for assistance, so hope I can get it sorted soon or they will be really fed up with me!)