NattyFlightOk so I did the light test and it seems to be that A and C are yellow and B and D are green which would indicate graphics card problems, so where shall I go from here?
That's what I suspected. First thing is to power off, unplug and press/hold power button for ~15 sec. Open the case and carefully reseat the video card in its slot.
If that doesn't help, your only choice is to buy a new PCI-e x16 video card because the D8400 doesn't have on-board graphics. Only you can decide whether to put $ into this old system or toward something new.
I really shouldn't be commenting about the age of your D8400 because I'm typing this post on my D8400 