Alright, I purchased parts to build myself a new computer recently and things were going just fine... until a little while ago.

Firstly, here are the parts involved:

Arctic Cooling Freezer 7 Pro CPU Cooler
Intel Pentium 4 531 Prescott 3.0GHz LGA 775 Processor
SONY Black IDE DVD-ROM Drive
Seagate Barracude 120GB 7200 RPM IDE Ultra ATA100 Hard Drive
ASRock 775Dual-VSTA Socket T (LGA 775) VIA PT880 PRO ATX Intel Motherboard
Patriot Signature 1GB 240-Pin DDR2 SDRAM DDR2 667 RAM
APEVIA X-Infinity Black Steel ATX Mid Tower (w/ 420watt PSU)

Continuing on, last night everything was swell. Hadn't got it fully up and running yet but I could go into the Bios and all that.

Today, I turn it on, the case LEDs and everything else start up for less than a second, then it all shuts off. I try turning it back on again, nothing happens. I unplug it from the back, plug it back in, hit power, nothing happens.

So that's about the short summary of it, it doesn't turn on anymore and I barely did anything between last night when it worked and now when it doesn't. But, I did do somethings.

Things I did between when it worked and when it didn't:
*Took the HD and DVD-ROM off the same cable, put the HD on the old cable, and the DVD-ROM on a new IDE cable.
*Put a jumper on the HD to make it a master (it didn't come with a jumper when i ordered it)
*Moved the jumper on the DVD-ROM to make it a master
*Used three cable ties in various areas to keep the mess of wires away from the cpu fan.
*Gave power to a side fan which I left unplugged before as a matter of convenience.

Any ideas on what I'm looking at here?

Check that everything is plugged in as you could have dislodged some wires in side.

Also have you got another PSU you can try in it?

It could very well be your motherboard.

Make sure the power switch is plugged into the motherboard...i had a similar issue with my computer here - had to replace the motherboard.

I often find that undoing everything I did and then doing things one step at a time improves the troubleshooting process. But as long as everything is tight then something might be fried. Good luck!