Apple ibook G4 - close lid but won't sleep!

My computer ibook G4 shuts down itself automatically everytime when I either close the lid or put it to sleep. It used to be fine but now I have to push the power on to restart in order to use my computer. If I leave the lid up, it shuts down itself after a while anyway. It is realy annoying. What's wrong with the system? Is the problem related to the networking in the system panel. I just changed the length of time for the computer to remain connected when the internet is inactive.