You need a looking glass to have a look at the MOBO bottom right corner of board plugs should be marked, the main one is the power lead all others are just for lights more often than not even the Hard Drive led dose not mater it will not stop it starting the computer. But you have to get the power plug on the power pins, these will be marked on the MOBO by the pins. for the other plugs you may have to revers plugs to get them to work but the power plug will not make any difference it should work ether way round. If it’s a new build do not have any drives connected until you get something on screen white letters on black background.
I'm getting absolutely no response from the board/system at all (no PSU or case fans starting, no beeps, nothing on the monitor, nada)
Yes, the front panel connector leads are labelled?
I have the mobo manual, but had difficulty 'translating' the connections, especially as the actual mobo pins aren't clearly marked, as I've seen (the one time I did this) before. I thought I had it right, but...........
It's a re-build - replacement mobo, processor and memory, the last mobo having blown a chip, I was told, probably by a power surge.
I also have a replacement PSU, which I bought when it died, assuming that that was the problem - haven't tried it yet, but also will.