There are 2 reasons I can think of that are fairly likely:
1. The DC in power Jack is screwed, or your power cord. I've replaced the jacks on a few laptops. The jacks are cheap and this happens a lot on many brands.
2. Your laptops charging circuit is dead. In this case it is usually a chip or chips on the motherboard that controls the charge to and from the battery.
less likely:
3. Your power cord to your adapter is unplugged...