No beep, No display, what wrong is my PC?

I just fixed a computer that had the same exact symptoms. It had all started when a friend wanted me to transfer the guts of on pc to another. Everything worked fine before I started but when I was done I was in the same boat. First thing I noticed was that the Power button would start it but would not turn it off after pressing it seconds at a time. I didn't think much of it as that is a minute problem compared to the problem at hand so I forgot about it. Down and out because nothing on the internet worked. I narrowed it down to about 3 components that could be the cause. The Motherboard, CPU, or Power supply unit. There are others but i was able to eliminate them. Now back to the power button and why the pc would'n't turn off holding it down. It took me some pondering but I came to a conclusion that this was most likely a power supply problem but that isnt always the likely cause. I got out the voltmeter and started checking the power supply unit. I got to the 8 pin ATX Power connector on the Motherboard and had no power from it. Now I have looked up the symptoms of not connecting that power connector to the MB. This may include, No beep on startup, no video, fans and lights come on but nothing else doesn't. The wire actually had a loose connection at the Power supply. The fix was actually easy. I took that cable off and checked continuity of each wire and I didn't find anything wrong with it. There are a couple fixes, 1 buy a new cable or power supply depending if you can remove it or altering the wire to fit in the plug. ONLY do this if you know what you are doing and experienced. I am not responsible for what you do to your computer. I altered the wire because my friend wanted it up and running ASAP without having to order a part. To do this I cut the end of the plastic connector( the side that is most loose. Both sides may need it.) I took off about the with of a dremel cut disk. this allowed me to get a better connection with the pins. I plugged it back in and noticed it had snugged up quite a bit. Turned it on and it all seemed to work along with the power button now when pressed Turns it off. Sometimes it is finding those minute problems can be all the difference between fixing it or junking it. Check all your connections for looseness, have patience and eliminate items you know for sure have nothing to do with it. Don't give up or get discouraged. There are solutions for this issue

Yes, the power supply fails, and I replace it, and it works fine now.

Thanks everyone very much for suggestions

You probably need a new CMOS battery. But i would advice that you troubleshoot all the other scenarios, mentioned in the link below.

http://www.ehow.com/about_5374778_cmos-battery-problems.html

Good luck.

I get a business trip for 4 weeks, before I go, I unplug all power cable and supply for my PC, when I come home, and plug-in all power cable and supply, without turning on my PC, the light on USB hub is constantly flashing, which is strange and it never happen before, after that, I turn on my PC several times, but nothing come out on screen, not even seeing the BIOS.  I wait for 10 minutes, and turn on my PC, the issue of flashing USB hub is gone, finally I can see my XP startup, at this moment, I decide to restore my system image using Norton Ghost, during restoring process, it fails, and never happen before, after that, I turn on my PC several times, but it is no beep, no display. when I open my PC case, and turn on the PC, the light on motherboard is on, and the CPU fan and fan are running, it is still no beep, no display at this moment,  This is my second time happened, after I unplug all power cable and supply for 3~4 weeks, and plug-in the power, something strange is happened on my PC. Does anyone have any suggestions on what is going on for my case?

Thanks you very much for any suggestions