How can i fix my nintendo ds lite?

My Ds lite has been lying unused for months and when my daughter tried to use it, it wouldn't charge. I bought a new charger and when plugged in it looked as though it was charging as the light was orange but when it should have been fully charged with a green light showing the lights all went out and when tried to turn it on with a game in it, it would not do anything!! Do you think it could be the battery? And could it be replaced easily and cost minimum? Please help!!