POST. BIOS and Troubleshooting

Source

Power On Self Test

• The BIOS determines if the video card or onboard video is working.
• The BIOS verifies RAM.
• It checks for a keyboard and mouse.
• It looks for a PCI bus and then checks the cards that are installed.
• The system information is then displayed on the screen.

POST is determined by the computer's BIOS (Basic Input/Output System)
User settings are made in CMOS (complementary metal oxide semiconductor)
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Fault: Computer will not start
Fix: See below
Disperse internal Voltage

• Remove all power (adapter power Cord and Battery (for laptops))
• Hold down the ON button for 30secs (dispersing internal voltage)
• Plug the power cord in only (ie leave battery out, for the moment)
• Turn on

Did it work?
If so, shutdown normally, and fit the battery back in (for laptops)
Continue on as per normal

If not, try removing all externally connected devices, leaving in Power; Video; and Keyboard only 
(even test without Keyboard and mouse plugged in)
Reset CMOS

Many No POST failures are caused by your computer's CMOS settings being corrupt
Resetting the CMOS to defaults will cure this, and possibly allow your computer to POST
By reading your computer's or motherboard's Manual (discussed further down this page)
Locate the CMOS battery and/or the "reset CMOS jumper" on your motherboard
Following the reset CMOS procedure (usually shorting two pins, with CMOS battery out)
Your CMOS settings will be reset, hopefully allowing your computer to startup (or POST)

Hardware issues

Firstly it's always best to download your computer, or motherboard, manual
You can locate your manual, by going to you manufacturer's support page
For convenience (and as an example) Here is Dell's and HP's support info
Make sure to follow all AntiStatic and power recommendations/precautions

On PCs: (ie not laptops)
Re seat the Ram card(s)
Reconnect all internal connections 
All drive data cables can even be fully unplugged
Re seat or remove, all addon PCI cards
Unplug the case fan
Or try Benchtesting the motherboard
On Laptops:
Please note, it is also a good measure to re seat the Ram card(s) and also the Harddrive for laptops
You may need to locate your disassembly guide, at Google, not all manufactures provide this guide.
As an example, here is a good Toshiba laptop disassemble guide
Removing Dust
Dust and debris, is usually the biggest factor on computers that will not start
Using High compressed air, or a vacuum cleaner, remove all dust from the computer
With special attention on all Fans; the CPU; the Ram; and the Power Supply Unit
Here's an excellent guide to clean your computer
Power Supplies

Note: Although this subject has been discussed in the "Benchtesting" link above,
Just because some lights come on, does not mean your Power Supply is working properly.
It is also possible that your actual power switch is faulty
Or even the Display off micro button is faulty, (on laptops)
In most cases, it's a matter of testing with a known working Power Supply
Here's an excellent link on Power Supply information and Troubleshooting

Some more helpful links
Read the original No Post Tutorial
Beep Codes
Ultimate Boot CD
Reseating your CPU
An excellent Flowchart
Windows Repair Guide
Manufacture Drivers
Drive Diagnostics
Memtest

If you still require support with No POST, please create a New Thread
Stating what you have tried (or reply to your own existing related thread)
Making sure that you put System Specs information in your Profile