How can i fix my Dell Latitude D600?

Last year i switched off my laptop properly and put it away, the next day i wanted to use it again but found i was unable to switch it on. I plugged in the charger and the green light on it was on so i left it to charge for a while. I found that my laptop would still not switch on and so i assumed it was the battery. A few months ago i left it on charge all night to see if it was the battery but in the morning there was no change. Now, i really need to use my laptop but have no clue as to how i can fix it. Has this happened to anyone else and does anyone know how i can fix this?
Thanks in advance.