often this is caused by the power jack itself being broken. dropping the laptop, accidentally kicking the power cord while it's plugged in and other abuses can break the internal jack and cause it to not work.
i'd suggest taking it to a local tech shop or ask your neighborhood tech geek to have a look at it. not much i can do from here without being able to touch it and check it out myself. the jack can usually be replaced and they tend to be less than $10 for the part. but the labor will cost a bit since it involves taking the laptop completely apart.