I'm having touble with a compaq presario s4020wm. I see one light comming on but the other one never does . The power pack light is green and the fan is running also the fan on top of the prcessor is running but, the computer never comes on. Also the computer monitor never displays it goes to starting up as a green light then goes to a yellow color which mean there's never no screen on it. Could this be the motherboard has gone bad ? I even tried to hold the power button in for about 30 seconds to see if it would reset then boot up but, it didn't work. I also tried removing the battery and then replacing it didn't work to get to the BIOS. I tried the two jumpers by the battery and it still didn't work. I know a great deal about computers but, this one has me even puzzled. I do hear a slight sound of a beap comming from the computer but, it's like I said it could be the motherboard.

the green plastic "panel" as you call it...may have a clip that's securing the hard drive in it. see if there is a tab you can pull up (gently) with a finger that may release the drive

I had the same problem with my computer and the Compaq-HP techs told me to pull the ram out and then put it back in. They called it reseating the ram.
This fixed the problem i had. Try this and good luck.

you may have problem in your motherboard take it to workshope.

I suggest you have Power Supply problems. A failing power supply will emulate all kinds of software problems.

Let's start with the easy stuff first.

How dirty is the computer inside, and the Power Supply? Has the inside of the computer, and the Power Supply, been cleaned on a regular basis?

(Computer off, and unplugged from power, computer case open. A can of compressed air is used to clean)

From there I would suggest testing the Power Supply, (Requires a multimeter, or a power supply tester), or replace the power supply with a known to be good unit, for a test.

To summarize:

Let me know if the computer is dirty inside, and if you need guidance on how to clean it. (Power Supply also)

From there, if you wish I can guide you on testing your power supply, or replacing with a known to be good unit, for a test.
Simply state in a Comment.

This is HP Support, and the download page for a manual which instructs on how to open your computer case, (Page 118)

With this manual I can also guide you in replacing the power supply.

http://h10025.www1.hp.com/ewfrf/wc/manualCategory?lc=en&dlc=en&cc=us&lang=en&product=326832&

Why do I think it's a power supply issue?

You don't have enough power to turn the Processor on.
Power Supply's are rated in Wattage.
Voltage times Amperage = Wattage.

1) ALL the lights require less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts of power.
3) A typical Processor requires 51 to 125 Watts of power. Depends on what processor it is.

The Compaq Presario S4020WM Desktop PC, comes with an AMD Athlon XP 2400 processor. (2.4GHz)
When operating at full capacity, this processor can use up to 68.3 Watts.

http://en.wikipedia.org/wiki/List_of_AMD_Athlon_XP_microprocessors

You have enough power to turn on lights, and spin fans, but not enough to turn the Processor on.

I think it would be much easier for you to use a USB hub instead.
It's much more easily available .

You can connect the hub to one of the rear USB ports

Pull out the power cord for 20 seconds, then try again.
Power strikes can lock powersupplies up and give those exact symptoms.

Ensure that the power button is pressed and the monitor is plugged in correctly. If ok, then the installation was stopped while installing your video drivers. If possible use another monitor to re-load the xp operating system until completed. Once the video monitor drivers are loaded the monitor will work again. Let me know if you have any questions.

Don't forget to rate this solution before you log off.

Check here : http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?rule=39140&lc=en&dlc=en&cc=au〈=en&os=228&product=326832
Click on HP Home on taskbar to find full menu of services.
Cheers.

you mean the internal power supply? that's available for around 20 bucks anywhere online..usually called a PCXT power supply..big issue is finding one that's identical with connectors..

take yours out, and then maybe there will be a part number or maker on the label.

Actually you do want to hear one beep.
This is POST. Power On Self Test.
No display is a video problem. Swap the card for a known good video card. You should be able to get into the BIOS with that. Presario’s are quit.
While in the BIOS see if the HDD is shown. It doesn’t hurt to check the connection inside. Please keep yourself grounded when inside the PC. For the Compaq Presario S4020WM site go here:
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?lc=en&cc=us&dlc=en&product=326832&lang=en&