CPU starts but monitor display no signal

Hello,
I have a problem on my computer. few days before the computer runs normally and now this day when I turn it on the monitor displays "no signal" the cpu fan is working fine, the videocard fan is also working fine, the casing's fan is also working fine but the monitor's LED just light in blue then after a few sec it turns to orange and displays "no signal" I don't know what happen or what could b the problem. I tried swaping the memory card with my new one but still don't work. I unplug keyboard, Mouse and Harddisk but stil displays no signal. if I remove the memory card from the slot it beeps which means the memory card slot is working fine. Please someone could help me with this problem