what drivers are you using, what condition is the Cables at hopefully new as a common issue can be cables getting old. Amd cards are very picky with cheap cables I found with my own experience, another thing is make sure you got a decent PSU way over wattage could be your PSU not putting out enough juice after boot to drop 1 monitor. I suspect either PSU or cable issue if everything was working fine before. Another idea is reinstall your OS to suit with fresh Bios & video card drivers could be a corrupted update from switching from intel to AMD. I suggest to reinstall windows first then check all cables & PSU as well run a power usage app to get the true value of you system full power draw.