I've seen this happen 3 times in past week, system is idle, screen goes black & says no signal. Disk activity light is on, solid, don't hear any disk activity. The reset button or power button or ctrl-alt-delete will do nothing. I have to shut the power off on the PS, turn it back on then system will restart ok. Build is 3 years old & has been problem free till now.
Check the connections from your case to your MOBO. It sounds like none of them are connected. I'm not sure about your video troubles, though. I assume from your specs that you aren't using a discrete GPU?
I guess I was unclear. PC works normally, case switches work normally. Then, at some unpredictable time, screen goes black, pc is frozen, fans still running and at that point the case power & reset switches don't work. I have to power cycle at (now new) power supply & pc will start up normally, for a while. There is nothing relevant in the event viewer, temps seem normal. I'm thinking now it must be the mobo.