Problem solved - Windows 10 had the only use one display option selected. It must have been chosen after turning off clone mode. Pah!
I have a new pc with an RX580x card. I have been using it fine with 2 monitors, one DP and one hdmi. I went into Radeon software and turned on clone mode, and both screens showed the same, turned it off and my second (hdmi) screen went black. I powered off and on again, no difference. I thought I would swap cables and see if that made a difference, and when I unplugged the DP cable the other monitor immediately sprung into life. I plugged the DP cable back in and the other screen went Back to Black (thanks Amy!). Any ideas as to what is going on, and how to get both my monitors working in extended desktop mode? Edit - I turned on the AMD Eyefinity and both displays worked together as one wide one - unfortunately I can't use this as the displays are too far apart, I need them to be seperate. When I turned it off I went back to one display only. Windows 10 display settings only see one monitor.