MA790X-DS4 CPU fan not working

Hello people,

I've just put together my first home built PC. The specs are as follows:

Motherboard MA790X-DS4 Gigabyte
Processor AMD 6000+ using stock fan
Graphics 3870 Sapphire Toxic
RAM 2GB Corsair
Power VX 550W Corsair
Hard drives 2x 500GB Maxtor
Case Thermaltake Armor + MX
OS Vista 64 Home
+ a wireless card and an IDE CD/DVD drive

Please have mercy if I've missed something really obvious, I'm completely new to this. I did read the manuals and forums before putting everything together and have double checked the connections, but am still clueless to what's actually causing the problem.

The problem is the CPU fan is now not turning on at all when I boot the computer. I had the same problem when I first put the computer together, but after checking all the connections it started working without me realising exactly what I did, if anything, that fixed the problem. Anyway, I installed Vista 64 and even got Crysis up an running, everything was fine. Then I got home today and tidied the cables up with some zip ties and when I put it on again, the CPU fan won't turn on just like before. I've looked at the BIOS settings and even tried the four pin connector in the SYSTEM FAN 2 socket, but it just won't turn on at all. I've checked all the connections on the motherboard but I can't find anything that has changed since yesterday.

If anyone could point out what I'm doing wrong here I'd HUGELY appreciate it. I'm starting a Masters in Computer Science tomorrow, and I'm not seeing this as a good omen!