I built a new desktop and there's no signal to the monitor.

Hello,

I've spent the day making a new PC and now, after many hours, all the bits are in position and plugged in. I plug the monitor I've used previously for an extension to a laptop in using a VGA plug and then it'll scan for either a VGA or DVI port. i have one of those remember, but then it says it can't find the signal and goes into sleep mode.

Smart people of the internet, please help!