Try the usual. Unplug the HDMI cable and remove power from the HDMI display for 5+ minutes. Plug in, power it up and see if it continues. If it does, it's 99.99% sure it's our old HDCP issue. There are folk that will sell you HDMI cables (don't buy!) And I take it you tried checking this unknown monitor's native resolution. I had one a few months back they were sending 1080 to a 720 display. What a mess. Poor client did say "it worked before." My answer is that it shouldn't work at all and if it did, probably going to fail.
i have 2 monitors one with HDMI and one VGA my hp with HDMI goes black for 1-2 seconds while im working this happen only on HDMI monitor what is the problem i tried to switch HDMI on monitor that is with VGA but same problem