Which is the correct assembly for the power supply to the arduino-ramps board?
power brick to arduino-ramps only
computer power supply to arduino-ramps only
power brick and computer supply to arduino-ramps
usb to arduino-ramps only ?
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Power only to the green plug! You can use a USB at the same time if you want but the only other power should be in the green plug.
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opps 
ill need a new board i think 
Depends on how long you powered it wired wrong… Does Windows recognize it when you plug the USB in?
If not, you probably have a couple damaged parts, but they are fixable…
If you plug the arduino into ramps and power it for a few seconds, does the regulator chip get hot? The regulator is in the corner near the power plug… And are there any lights in the arduino?
Mostly what happens is you burn up the regulator and you burn a safety diode on the USB. The regulator is simple to replace and you just remove the diode although it is also simple to replace.
Test and let me know… Then I can tell you how to replace the regulator. (You can get them from Amazon, or aliexpress, or any number of places.)
I just spent most of today dealing with this same exact issue. Once I powered the ramps board with the two wires cut from the power supply things started working much better. 
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Yes on windows recognizing it when using USB only
Nothing gets hot when arduino and ramps are plugged in but the multimeter still shows power going to drivers (test with the 12v power brick to arduino only, and test with 12v computer power supply only to green things only) Also there are no light indicators with either of them plugged in… anymore
If you plugin the board with USB only, all lights come on. I think I blew the big yellow rectangular things on the ramps board?
Whats the resistance across the yellow fuses? Check the voltage at the screws of the green connector.
Sounds like the 5v regulator.
It would be the first time a regulator failed open that I’ve seen. They usually fail shorted and get hot…
so, if we assume Ryan is correct, you can verify this by putting about 5 volts into the barrel connector on the Arduino and then verify that there’s no voltage on any of the small pins of the regulator. be careful not to bridge the pins when you measure voltage on the regulator or you could blow it up…
if leslie is correct, you can measure resistance across the yellow fuses. it should be near zero.
the way I troubleshoot this when something shorts is to put correct voltage into the input (Green connector). measure voltage at that point (and current if you are able), make sure it’s in spec. then move to fuses, then look at the diode just past the fuses. and start working your way through the circuit to see where voltage goes away.
things that tend to die for me in rough order are: arduino regulator, arduino diode connecting the regulator to usb (at least, I think it’s a diode), ramps fuses, ramps diodes that bridge boards (its underneath the x axis driver). and, to find it, as I said, I chase the voltage around the board to see where it quits.