Black screen only after I turn on PC,help

I changed power supply unit few days ago, and now the voltages are OK.I had low voltage problem when I plug my pci-express card and it worked fine with new power supply unit for few days till yesterday.A month ago,strange problem occured,and yet unsolved.Today and yesterday I can't power ON my computer.I see only black screen on monitor (the LED on it blinks),I don't see MB logo screen before it boots up,nor HW detection,and red led on chasis remains red all the time.The problem occurs quite often,yesterday my PC worked fine,I switched it off for a 1:30 h and tried to turn it on,but no luck.I tried everything,switched off HDD,unplug memory,changed display adapter,still no luck.It gives me conclusion if it's motherboard faulty.I even unplug CMOS battery and plug it again.No luck.
My motherboard is asus M2NPV-MX,I'm having 2 HDD (sata and pata),and 2GB of DDR 2 Kingstone memory.

I can't remember old one,it was the one I got with chasis.New one is Trendsonic 400W model (cheap one).
My graphic card is Sapphire ATI 4650 512MB DDR3,and on-board graphics is GeForce 6100.