The power jack is physically connected to the motherboard.
If the power jack has broken from the motherboard and\or the jack itself is toast, it can be repaired but the motherboard has to come out to do it. To complicate things a little more should the power jack need replacing, Dell uses a special 3 conductor power jack. Your better off taking it in and having it repaired.
If anyone tells you that the motherboard needs to be replaced without even disassembling it first, walk away. It shouldn't really cost you more then $100.00 labor plus the cost of the jack if it needs replacing.
If you want to see what's involved in dis-assembly, the service manual for the 5150 is here.