Which system are you using?
Select... MENU / Wireless / Find Recv - The Controller will search for and link to the Receiver (presuming the receiver is powered and functional - indicated by a lit LED. Orange for idle, Green for connected)
Attach the Controller and Receiver together with a motor cable. They should establish a WIRED connection, and synchronise settings.
After 30 seconds, disconnect the wire and wait. They should re-gain their Wireless connection.