Monitor on Standby :(

Hello,

Straight to the problem. I recently changed my power supply and added a video card to my system, from then my computer switches on (meaning the power led's are on, tower fans and the power supply fans spin as usual, i can feel the hard disk working, there is even a light on the mother board) but the monitor remains on standby. But when i remove the signal cable i get the message "check signal cable". Before replacing the power supply, I had cleaned the computer using a low power vacuum cleaner with air blowing out.

Things I have tried to fix the problem:

1. Checked the monitor on another system (working fine)
2. Removed the Video card and connected the monitor to the onboard video card (Still did not work)
3. removed both the ram sticks and turned on the system. (heard 3 beeps, so i am hoping the mother board is fine).

Computer Configuration:

Processor: Intel Pentium D (Dual Core)
Mother Board: Intel Essential Series (With onboard graphics)
Ram: DDR2 (1 GB * 2)
HDD: Seagate
Power Supply: VIP 230 V (I am in India)
New video card i tried: XFX Geforce 8600 512 MB (PCI express slot)
OS: Windows XP SP3

PS: I have removed the video card currently

Please help!