How to fix my Nikon D40 Camera?

I have a Nikon D40 camera and its in great condition but when ever i charge the battery and put it in to turn on the camera, the camera doesn't turn on even when the battery is fully charged. Is there a way to fix this?

Try a new battery, and if that fails, contact Nikon.

I agree, try a new battery. You can contact Nikon but at the very least you should have a pro look at it.
It'll cost you to find out what's wrong but it's probably worth it.