|You are going to want to open this computer up. |
check the voltage at the mainboard where the DC Jack cable attaches. This can rule out some things. If you get correct voltage at the mainboard, should be 19v, then there is a problem with your mainboard.
If your don't get any voltage, check the solder connections at the DC jack. You will have to peel back the shrink wrap around the wires and check there. If they are good.
Check the continuity between the pins of the DC jack and the end of the cable. You should get very low resistance. If it's high, then check the resistance between the pins of your DC jack and the leads of the jack where the wires are soldered. If it's high you will want to replace it.
The only other thing would be bad capacitors or MOSFETs. Or it could be the NEC / Tokin cap.
Hope this helps.