Computer starts but monitor says "no signal"

Hi,

Yesterday, my monitor worked fine. Today, l connected it to a different computer just as a test to diagnose the monitor problem on that computer. This monitor again functioned properly on the second computer. Then, l brought it back to the original computer and reconnected, found everything as normal.

When l turn the computer on, the monitor shows the brand name screen and then goes to a black screen with the "no signal" message.

I know the computer is functioning because l can hear the welcome sound even though nothing shows on the screen.

I have tried unplugging/re-plugging and removing/replacing the CMOS battery. No success!

How do l get my picture back?

Thanks!