Laptop won't charge and green light won't turn on

I know that the charger and outlit works because I plugged it into my sister's computer and the green light turned on and it was charging. But when I plug it in the light doesnt turn on and it doesnt charge my computer. I think it might be the battery but i have only had my macbook pro for a year... All my essays and papers I have to turn in are on my laptop and I definitely can not afford another one... HELP!!!