Couple of possibilities, but the most likely scenario seems to be that the motherboard died. Still, may as well try a couple of the simple free options first. Pull the power cable out from the computer and leave it out for a good 15-30 seconds to make sure the PSU capacitors drain. You can press the power button as well to help facilitate this. Then plug the computer back in, give it 5 seconds or so to energize the capacitors, then see if it will boot.
If it still doesn't want to boot, odds are you're looking at a bad motherboard. The PSU is a possibility, but generally if you have a bad PSU, you don't get any kind of movement when you press the power button. I'd be willing to go so far as to say the symptoms seem to fit the bad caps (google it) issue pretty well. For most people that's a death sentence for the motherboard.
What I'd suggest you do is get your brother out to take a look. He built the unit so should be reasonably familiar with its component parts. If nothing else, odds are he'll have an external enclosure you can use to pull the HDD and get the files you need off the drive so you can finish your work on a campus computer or something.
So my computer was working fine last night but at some point when I tried using it I realised the monitor was not picking up the computer, so I turned everything off and tried again this morning.
Once again the monitor is turning on fine and the computer lights up but the monitor is not picking it up and the screen stays black. I noticed that there is also no power going to the mouse or keyboard and there is no beep on startup. Having tried booting the computer about 30 times the computer is now turning on for a second and then going straight back off.
I took a look and all of the fans are working properly when it does boot but unfortunately my brother built the computer so my knowledge kind of stops there. I tried booting with only the power cable and that also didn't work. I have easy access to the inside of the computer but have no idea what to try. I have exams coming up soon and all of my notes aree on the computer so would appreciate any help. Cheers.