Monitor doesn't work and USB devices aren't detected

Okay, so I'm helping a friend out in building his first computer but the problem we're facing now is that the monitor connected up to the computer (that's finally put together) doesn't show anything, and the USB devices (being the keyboard and mouse) connected don't work.
If there's anything you'd like to ask about the build, what's connected what's not, feel free to ask; I'll tell you what you want to know.

*Things I know for sure*
The mouse and keyboard aren't broken.
The monitor worked 2 days ago, so I highly doubt the monitor just stopped working all of a sudden?

*UPDATE*
So... my friend managed to get it working by himself, but what bugs me is that the problem was with the RAM.
There are four ram slots; so RAM Slot 1, 2, 3 & 4.
1 & 2 are the first 2 rows of RAM that's closer to the CPU slot/cooler, and the cooler was already installed (Cumbersome)
So decided to use RAM slots 3 & 4 instead.
For some reason THIS was the problem; what had to be done at the end was RAM slots 1 & 4.
Slotting in the RAM with any other combination other than 1 & 4 didn't allow the computer to recognise the monitor or the USB devices for some reason.
Although the problem is solved now I would still like to know what went wrong, I want to learn from mistakes.

That is so strange, I never knew there was a specific pattern in which the RAM was supposed to be installed.
I've always figured you could just use any slots in any order (although strange to be random with the RAM's placement for no good reason), and just to use which seems suitable be it 1 & 2 or 3 & 4 etc.
Anyways thanks I guess? haha