You may have tried all of these things but just a quick list here of things to try:
Make sure you have the computer unplugged from power while removing or moving any hardware...
Try 1 stick of ram in 1 slot at a time (remove all other ram sticks). Move it to the next slot until you have tried that stick in all slots by itself. If you still have no post, try a different stick of ram in each slot by itself. If you have issues with the stick in 1 particular slot (a no post for example) the slot may be bad. It is also possible you have a bad stick of ram instead of the slot being bad, in this case the ram stick should cause the same problem all slots.
You may have a memory controller issue in which case if you have 4 slots, 2 may not work. With intel it should be 2 slots next to eachother channel 1 might be slot 0,1 and channel 2 might be 2,3. Trying with 1 stick at a time in all 4 slots is the best way to test everything.
Try resetting the cmos, remove all power sources including the main battery and then remove the button cell battery from the mainboard, leave it out for at least 10 seconds. Then put the button cell battery back in and power on.
If you still do not get a post screen:
Disconnect pci cards including mini wireless cards ect, pci-e cards, hard drives, dvd drives, disconnect all usb headers/connections as well, webcams etc. You should only have the power supply, main board, 1 stick of ram in 1 slot, the cpu and cpu cooler connected. You can also plug in a monitor and keyboard. Now try to power on the machine. If at this point you have no post screen or video etc, unplug the computer and pull the stick of ram (you should have no ram now), power back on and listen for a post fault beep code.
If you get no response next: remove all of the components from the computer case and connect only non essential hardware outside of the case, place the motherboard on a non conductive surface, a wooden table will do fine. Then try to power on again. Sometimes a screw will come lose and get stuck under the mainboard and cause a short.
Generally the issue you are reporting is a video chipset failure. Replacing the motherboard might be the only option. Double check that you can not see anything on the screen with a flash light held against it, sometimes backlights fail. If you get no post splash screen on either the external or built in display at this point, you likely will have to replace the mainboard.