|If we assume that the computer fully powers up and there is no video at all at any point then we might suspect video card or even video settings in bios.|
Power down everything and remove ac plugs. Press power button a few times and then reseat the video card maybe if it is an add on card.
Might try to reset bios if there is a jumper. Again the power deal and move jumper back and forth once to reset bios to default.
I would not waste my time with an upgrade. OEM software won't work on a new board unless you bought an exact emachine motherboard. Get a new system may be just as easy and fast. Good home systems are not that much anymore and maybe you can use some of the parts from this.