I've been googling this problem for a while now but its hard to come with a good search result because Its hard to describe whats wrong and the symptoms.
One morning I wake up and start my PC. I went to take care of something else so I didn't see when it was booting up. I did however heard windows welcome sound. When I went back to the room the TV was blank with the message "No Signal"(I use a Sony TV as my PC monitor). I made sure the TV was on the right source and then pressed the reset button on the PC. After that the PC would not boot, no beep codes, nothing. The fan do come on but with low power intensity. The LED lights on the fan and some other devices that have some sort of lights do come on but not at full power and flickering, (a fast pulse-like flickering). I still don't know the cause for this. When I leave it be and a couple hours have past, it does come on normally, but at certain point in the booting process it shuts down and comes back up with the same symptoms. I know every other component is fine because sometimes when it has time to complete the POSTing process everything works fine.
My system specs are as follow:
Asus P8P67-M PRO
Intel Core i5 2500k
Corsair 4Gb of RAM 1600Mhz
Corsair CX600 PSU
eVGA NVIDIA GTX 260 (used, I went cheap on the GPU I know. A new one is coming).
Cooler Master 212+ heatsink
Corsair 64Gb SSD
2 old HDD 300GB each
HAF 922 case
At this point it could be almost anything. My first inclination is your GPU has gone south. Do you have another board you could replace to see if the problem goes away? I keep something like this on hand for just such a diagnosis: ZOTAC ZT-84MEG5M-HSL GeForce 8400 for $10 after rebate. Actually, mine is loaned to a friend right now for exactly the same reason. If you live in Atlanta, you can get it from him if you want, but I digress.
Failing that, you are in for the usual slug fest diagnosing these problems: "System won't boot" and "no video output" checklist. shortstuff_mc did an excellent job compiling the most common build problems. Not everything here will apply since you have already had your machine working, but this list still has a bunch of excellent suggestions. I recommend you print it out, and start crossing steps off.
If you would like to go straight to the heart, skip to just past step 22 and follow jsc's suggestions to isolate power supply problems. It helps if you have an extra one of those around, too.
If you want to look on the bright side, think of it as good practice un-building and rebuilding your machine!
Unplug all the connectors and then put them back together ensuring there are no loose connections.. The power fluctuations, though primarily hint towards a problem with the PSU, could be caused by any other faulty component also.. May even be some problem with the motherboard..
I forgot to mention that I did changed the GPU with a 8800GTS and the problem still. I dont have spare parts to test whats wrong with it. I did even unplug the GPU completely cuz I should get an error code but the mobo wont even go that far. I used the MemOK button to start it up but the DRAM LED just blinks (the blinking light still does flickers). My next test is to find another PSU to see what happens. I will keep posting here with updates. Thanx for the reply guys. I will start with the suggestions
I did unplug all non essential components and isolated the psu. It was in fact the psu. The psu powers up but doesnt work properly under load. I replaced it with a 800watt, +12v 60a, SLI and crossfire cert. This should be enough this time. Thank you everyone.