Probably a blown component on the motherboard or power board. It's the same on a Dell I have -- runs on PSU won't charge the battery.
Luckily I have a couple of other Dells which take the same battery so can charge one in them.
Unless you have done it before I wouldn't recommend trying to fix it. I always take the precaution of buying a wrecked laptop of the same type before trying to open a working laptop. That way I learn where damage can occur when trying to prise the thing apart. Secondly, gives me a source of spares.