My Samsung p400 laptop won't power on. There are no lights on the front panel, even when the charger is in there. I had the multimeter onto the power supply and that is fine. Battery shows 80% charge with the level meter underneath. If I leave the battery out and power out overnight, and then plug power in alone, there is a green light that slowly dims to nothing.

replaced broken slide switch and power knob

1. pull out the battery after that u put the charger
2. if the you have the same problem change the charger
3. if still the same problem see the charging port of laptop
4. if the problem is not solve check the power ic on mother board

Here below are possible causes and troubleshooting steps of laptop that can't power on, hope it may help:
Possible causes:

1. AC adapter failure.
2. Power jack failure.
3. Motherboard failure.

Possible troubleshooting steps and repair solutions:
1. Test HP EliteBook 6930p charger with a digital multimeter. It's possible that you have a failed AC adapter. If you don't have a multimeter, test your laptop with another known good AC adapter.

2.It's possible that power jack (socket where you plug the AC adapter) is broken. Remove the motherboard and take a closer look at the power jack. In some cases you can fix the jack by resoldering it on the motherboard. In other cases you have to replace the jack with a new one. Using a multimeter, test if the power goes to the motherboard through the jack.

Here below are possible causes and troubleshooting steps of laptop that can't power on, hope it may help:
Possible causes:

1. AC adapter failure.
2. Power jack failure.
3. Motherboard failure.

Possible troubleshooting steps and repair solutions:
1. Test the AC adapter with a digital multimeter. It's possible that you have a failed AC adapter. If you don't have a multimeter, test your laptop with another known good AC adapter.

2.It's possible that power jack (socket where you plug the AC adapter) is broken. Remove the motherboard and take a closer look at the power jack. In some cases you can fix the jack by resoldering it on the motherboard. In other cases you have to replace the jack with a new one. Using a multimeter, test if the power goes to the motherboard through the jack.

3. If replacing the AC power adapter and jack on the motherboard doesn't help, most likely you have a bad motherboard. Replace the motherboard.

http://www.chargerbuy.com/replacement-dell-inspiron-mini-910-30w-ac-adapter-19v.html

When we had that problem it ended up being the battery for our laptop the charger was fine. Maybe check your battery?

I'm assuming that you mean the charger is not charging the battery or powering the laptop?
You first of all need to check that the power adaptor is working ok, that it has a steady green light and not flashing, a flashing light would indicate power is failing to reach the motherboard.

You can test this using a multimeter set to 20 volts and probe the red end to the inner of the jack that goes into the laptop and the black to the outer

If the pin in the laptop is loose or sloppy, it will need replacing. You need to dismantle the laptop and remove the jack port, this is soldered onto the board example of the part here

1. Take the battery out of the laptop & try to turn on the laptop using power adapter only. If the computer turns on, then the battery is defective & needs to be replaced.
2. Make sure the power adapter is not defective. Test the output voltage of the power adapter using a voltmeter. Compare the voltmeter reading to the nominal value written on the adapter or the laptop.
3. Make sure the DC jack is not loose or broken. A loose DC jack will need to be re-soldered. A broken one will need to be replaced.
4. If you have another processor which is compatible with your laptop's model, try swapping processors & test to see if the laptop will work.
5. The power button could be bad
6. The motherboard could be defective & needs to be replaced.

This is common when the charger is not working anymore and the battery is almost dead. It has just a few seconds of life.
Verify that the charger is working. Plug it in and see if any of the light come one in the front then unplug. If not lights then it isn't charging. Even if the light comes on, you need a multimeter (12.00 at walmart) and put it in and check for the 24/28Vdc (read what the output is on the bottom of the power cord).

Sounds like the AC adapter cable has a break in it.

If you have a multimeter you can test it. I use a paperclip to get to the middle if my multimeter probes are too big for the hole.

If it's got good constant power then the jack on the machine where the AC adapter plugs in is probably loose or broken.
Sites like laptopjacks.com sells new jacks that you could get someone to solder onto your motherboard.

Sounds like the power socket on the laptop may have broken loose from the board. Very common. If you have a multimeter, try and check the voltage ont the tines where the main battery receives it's charge, with the ac adapter connected to the laptop. That will tell a lot. Good luck