Hi! Upgraded to Windows 10 and today my HDMI monitor wasn't working.. Then my VGA one wasn't either which left me with my main monitor. I have 3 cable types going to 3 monitors. Here is how I got it working again..
Uninstall AMD drivers inside of "apps and features".
Run CCleaner for browsers and registry. Save backup changes to registry when asked.
Install latest Win 10 Driver for R9 200 (in my case).
Go to CCC, Common Display Tasks. Click Extend Desktop.
Make sure Extend Desktop is active in Win 10 Display Settings also.
Unplug HDMI monitor cable to PC and wait 10 seconds.
Plug back in to "reconnect" the monitor.
Push the source button on the monitor.
Everything works again!