"When power is switched on, I can see a green light, which I think is on the motherboard. This is before I boot the PC."
This is normal, and found on all modern motherboards, and indicates power to the motherboard, and (usually) USB. The power switch on the front of the case puts the computer in standby mode, as does choosing shutdown from the Windows options. This is not the same as Sleep of Hibernate - it's like the standby on your telly when you turn it off using the zapper.
The motherboard will always send error info to the monitor if possible, so when you hear beep codes and get no screen, it's because it can't.
Possible reasons are:
PSU failure.
CPU failure.
GPU failure.
Total memory failure.
I believe that the MSN-E does not have onboard graphics. If that's the case, the first thing to check is that the Graphics card is fully seated. Often (really often) the downward pressure from the monitor cable will gradually rotate the Grahics card in its slot, causing the back to lift (that's why there's a clip at the back of the adaptor - to try to prevent this).
The very first thing I would do is open the case, remove the graphics adaptor, ensure that the contacts are clean, and reseat it. While the case is open, you might as well do the same with the RAM.