I am going to assume several things here because you did not furnish details, so correct me if I am wrong.
I assume that you are not getting the POST screen on boot; the power indicator light glows ok, maybe you can even hear the cooling fan spin up, but the the screen is dark and unresponsive. The HDD activity light is dark. Possibly you cannot even see the mouse curser on the screen.
That means that the power from the power supply is not reaching the BIOS chip to enable it to run the POST (Power On Self Test) feature for boot. That is a low-level hardware failure, and that can only be caused by one of 4 things: bad power supply, failed CPU, failed motherboard, or a failed or corrupted BIOS chip (extremely rare).
Off the top, I would suggest your motherboard has failed. CPU and BIOS chips rarely fail, almost unheard of in normal circumstances and your power brick evidently lights. If your indicator reads a fully charged battery, it would appear the wall adapter and battery are good. HOWEVER...if you have a seven year old battery and wall adapter, that is very old and they usually fail on a laptop before that long a time. Check it out very carefully.
Laptops are a lot harder to work on than desktops, and repair/replacement of components is greatly restricted. They are designed to be used and disposed of when they have problems. About the only things that can be replaced without a lot of work are hard drives, memory, and batteries. Getting replacement parts for a lap that old is pretty well impossible, you will be forced to use eBay, and anything you get will be already used. By the time you pay for used replacement parts (if you can get them and they actually work) you will have paid in labor and materials almost as much as a new laptop will cost. Not worth it.