Its a pre built PC.
CPU isn't relevant really cos he's connecting it directly to the GPU and it doesnt work.
OP you might want to post in the customer service section as well as here, in case there is any system specific advice they can offer you.
Is it that same cable you used for the other devices?
Any adapters being used?
Does the output work from the other connectors such as displayport?
Does the HDMI cable seat in the socket on the graphics card fully?
Does the connector look visually OK?[/QUOTE
I've tried 3 different HDMI cables, all are fine i'm fully confident the PC is fine also and my own devices definitely are. Everything connected properly, fans normal, no loose or disconnected cables on the motherboard i've never had any problems with a single product from OC before, no problems with cables/seating, or my TV's or monitor as everything else works via HDMI such as laptop, another pc, a PS4 Pro.. CPU is a 3400G which obviously has onboard graphics, best i've got so far is, after reinserting all my cables time and time again in different order,after turning the PC and TV's/monitor on and off at different times etc, simple steps.. it finally booted up randomly once after i repeatedly switched power on and off and reinserted my Windows USB and kb/mouse etc, so i managed to get windows 10 installed , switched it off and back on and the problem has just started all overagain, no signal.. I Then tried inserting my RX 580 and an older GPU to try those instead, both won't show either. So it has came on once and now it won't as before so it has to be a display on my end. I'm just puzzled as everything else i own displays fine on the 2 tv's and monitor i own without any issue at all.