Unplug both hdd's.
Remove the video card and plug the monitor into the mobo.
Power up.
Do you see the bios screen?
I have a Gateway Dx4860-Ur28 Desktop. The machine came with Windows 8 preinstalled. I did the free upgrade to Win 10. The HDD was getting full so I decided to upgrade the HDD. While attempting to change my 1TB HDD to a 3TB HDD, my monitor said it would restart. The monitor now see's no input. My monitor works with other computers. I bought a video card to see if that would work, it didn't. I cleaned & reseated the RAM, that didn't work. I removed the CMOS battery, waited a few hours & put in a new battery, still no input to the monitor. I replaced the power supply with a better one, no success. I unpluged the optical drive. I tried a different monitor that I know that works to no avail. The only thing that I could think thats left is the motherboard or the i5 processor. I can not find a motherboard anywhere online for this machine & I can't afford a new processor. What motherboard options do I have? or is there something else that I can try? I also attempted using VGA rather than the HDMI, still nothing.
With the power off, I put the old drive in an external enclosure, the new 3TB drive in my computer & had it boot from the old drive.
When power is turned on, the power supply fan, cpu fan, & case fan all work. I do not see any lights on the MB but perhaps there are not supposed to be any. The light on the case and the power button light works.
I ordered 8GB of new ram, if that doesn't work then what motherboard could I use to replace the one that is in there? and where could I find one? I am having difficulty searching to find a motherboard for the Gateway DX4860-UR28. What other Motherboards could I use with my existing hardware?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic