Gigabyte EP45-UD3P will not POST

Gigabyte EP45-UD3P
Intel E8400
G.Skill 2x2GB DDR2 800
Sapphire Radeon HD 4830
Western Digital Caviar Black WD1001FALS 1TB
OCZ ModXStream Pro OCZ600MXSP 600W
LG DVD Burner Black SATA Model
Antec 900 Case

This is a new build that worked flawlessly for about 3 months and died in the middle of a Ubuntu Linux session while I was watching a video and performing some system updates. The system simply shut down without warning and now upon pressing power button, the CPU fan does not start (although it moves slightly when powering the machine on/off), all phase leds (which supposedly indicate CPU load) in mobo remain on, and system does not POST. There are no beeps or video. All case fans and graphics card fan start normally when powered on.

I've tried removing motherboard from the case and unplugging all components but the CPU, fan/heatsink, and mobo and CPU power cables but got the same results. Tried removing CPU form socket and inspecting for any bent pins as well as the CPU socket in the motherboard but all seemed normal. Tried re-sitting CPU fan and heatsink to no avail, it still does not POST and all phase leds remain on.

Is this likely a problem with the motherboard, CPU, or another component?

Please help!

@zipzoomflyhigh, thanks for replying. I did think about that but decided to inspect the CPU and socket pins anyway since my symptoms are exactly like those of this thread: http://www.tomshardware.com/forum/ [...] first-boot . But I didn't really think that was the problem since, as you said, my build worked for 3 months before dying. So yes, you are right.

Aside from having spare components (which I don't) and going by process of elimination, is there a way to troubleshoot this and figure out which component is at fault? Would setting up motherboard/case speakers and listening to the beep codes help?