What cable are you using? Do you have another cable to try?
Unplug and replug both ends to ensure the plugs are fully and firmly connected.
Did you go into the monitors settings and select the correct input port?
Is the RX 480 fully and firmly seated in its' slot?
Right click an open area on your desktop. You should be able to access the RX 480 configuration settings. Check that all are correct for your setup.