Computer won't boot monitor doesnt turn on no

Friends , I need to fix my PC. Monitor is not showing anything, just yellow light. Video card is working , I checked it on other PC. I was thinking, Power supply is bad, so I removed the all the cards and have just one memory stick and also use built in video card on mother board. Again no video signal. Then I removed all cards ,memory card , detached all drives and hard drives. Turn on the PC , again no video signal. PC is just showing lights on front. And when I switching OFF the PC , it taking time to switching off, that tells me that power supply is ok. When I turn on the pc , its not beeping at all even there is no memory in mother board. So I plug in the head phone and turn on the PC , PC beeps constantly , that was good sign. But when I switched on PC second time, PC beeps but beeps only three times. And third time when I switched on PC , it never beeps.

Any idea what happening?
Please help.

Mother Board is intel D915GAG / D915PSY
motherboard has a 15 pin blue VGA connector