Well, in situations like this, it would seem that you would benifit from a CMOS reset. This is a simple operation of where you remove the little round battery (silver coloured) located somewhere on your motherboard (if you don't find it, it could be under the graphics card, if you have a long one).
Anyway, this would restore everything in CMOS to default, thus improving your chances of a successful boot. Keep in mind that this will reset everything you've tuned in BIOS, so if you have some special kind of boot-order e.g. you'll have to re-enter such information.
I suggest you should check the installation of heatsink-fan assembly and the cpu itself.
The computer may reboot due to high cpu temperature, if cpu hsf has not been mounted properly and has come off during use. If this is the case, the computer should boot up after it has cooled down. If cpu came loose, it never boots up.
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here: http://www.cwc-group.com/casp.html
You can turn on the PC by momentarily shorting the two pins that the case power switch goes to.
You should hear a series of long, single beeps indicating memory problems.
Silence indicates a problem with (in most likely order) the PSU, motherboard, or
CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.
If no beeps:
At this point, you can sort of check the PSU. Try to borrow a known good PSU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
This checks the PSU under no load conditions, so it is not completely reliable. But if
it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should
change to one long and several short beeps indicating a missing graphics card. Silence or long single beeps indicate a problem with the memory.
Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.
Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.
Thanks for all your feedbacks. I forgot to mention I have tried all your suggestions before I posted my problem here, and nothing worked.
Finally, I set my sights on what I thought was a less likely culprit and a seemingly innocent component - my OCZ 550W Power supply. Got another one (CoolerMaster 550W), plugged in and VOILA!!! It worked!! Just out of curiosity I plugged in back the old Power Supply - nothing. Confirmed it is power supply. I didn't think that could be the problem as there was power. Fans were running, LEDs lit, beep and all but POST.
This could be a helpful experience to those having a similar problem.