My Compaq CQ40 144tu is not charging when i plug in my adapter. This problem came on the fly without any damage to the laptop. Previously when I used to charge the laptop, the white LED used to be on where i plug in my charger. But now, the LED is just off.

The possible causes are as follows:
1
Determine if the laptop battery is defective. While less than 1 percent of batteries are defective, the possibility is still there. If the battery is new and only charges some of the way, it may just require a BIOS update to charge fully. New batteries that refuse to charge at all may be defective. Purchase a new laptop battery to replace the defective one.

2
Test the laptop power adapter to see if it's faulty. Run the laptop directly from the laptop power adapter and without the battery. If the laptop does not work with the adapter plugged in, the adapter is dead and you will need to replace it. The laptop battery may refuse to charge if the adapter is faulty. Charge the battery with the laptop turned off.

3
Insert the laptop battery into another laptop to test it. If the laptop battery charges in another laptop, the laptop's motherboard may be bad. Replacing a motherboard in a computer can be very expensive. If the cost to fix the computer is not worth the cost of replacing the motherboard, consider purchasing a new laptop.

4
Reinstall the battery to ensure all connectors latch properly. The latch mechanism that holds the laptop battery should click into place. If the latch does not close, remove the battery and examine the compartment for signs of damage.

5
Troubleshoot the BIOS to determine if there is a hardware problem. Turn on the laptop and immediately enter BIOS setup by holding down the "Del" and "F1" or "F2" keys, depending on the make and model of the laptop. When the BIOS screen appears, see how long it takes for the battery to charge down. If the battery does not last as long as the BIOS setup, a hardware failure may be the problem.

http://www.aurabuy.com/cheap-12-cell-battery-for-hp-compaq-presario-b3300-c300-c500-m2000-paypal.html

sounds like your DC coonector came loose from the system board.it is a common problem with laptops,

Problem symptoms:

1. Laptop randomly shuts down without any warning.
2. Power LED and battery charge LED start flickering when you wiggle the power cord or the AC adapter tip on the back or your laptop.
3. The battery will not get charged.
4. When you plug AC adapter, the laptop appears to be dead and there is no LED activity at all (DC-IN jack on the system board is broken).

here is how to repair:

http://www.laptoprepair101.com/laptop/2007/12/06/dc-power-jack-repair-guide/

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_8b8c2cd6ce148309

go to compaq's website, download the latest Bios Update, run the bios update while the power suply is connected on the laptop. after the update, the power adapter should be recognized by the laptop

Yes it is most likely your power adapter. If you know how to solder or know anyone who does the repair can be done yourself. Check out instructables.com for more help on soldering power adapter.

Some laptops will not power up with a completely discharged or faulty battery. Remove the battery from the laptop, plug in the power cord and try to boot the system. If the system boots, your battery needs to be replaced. Replace your battery as soon as you can, it's not a good idea to run the laptop for extended periods with an open battery bay.

Hello,
This is true, you will not be able to charge the battery without the correct charger. For mostly all laptops, when the charger is not maintaining a charge, it is time for a new one. However, it does not have to be the manufacturer's adapter, but I recommend purchasing the battery made for the Compaq Presario M2000 Notebook.
NextTag.com has decent deals on these adapters. http://www.nextag.com/ac-adapter-compaq-presario-m2000/search-html

I hope this helps. Please let me know if you have any other questions.
Thank You, Raph30

try removing the battery without the ac adapter plugged in then plug it in and turn it on hope that helps.

How old is your laptop? It may just be that you need a new battery if it won't hold a charge.

Further, this charger worked previously on this laptop and with this battery? If so, you definitely might be ready for a new battery.

Check the ac adapter is the proper one for that laptop. Check the power jack is not damaged or loose. let laptop site without battery or power for a few hours. remove battery and retry power. try another ac adapter if that doesn't work, you may have a reset button but not likely to solve the problem. sounds like motherboard is damaged, or something blew.

First off it?s a fairly risky proposition to plug a power adaptor from one device into another without making absolutely sure it supplies the correct voltage, amperage and has the correct polarity. Your laptop needs 18.5 volts, 3.5 amps, and approximately 65 watts to charge the battery. If you power adaptor is less that 3.5 amps it will shut down to prevent any damage so that?s probably what is happening. I would suggest going to the HP (now owns Compaq) or Ebay web sites and purchasing a replacement there.