The most common way that a computer "won't turn on" is when the PC actually does power on but doesn't display anything on the monitor.
You see lights on the computer case and hear fans running from inside but nothing at all shows up on your screen.
There are several possible reasons why your monitor isn't displaying information so it's very important that you step through an ordered process like the one I've outlined here.
Test your monitor. Before you begin more complicated and time consuming troubleshooting with the rest of your computer, make sure your monitor is working properly.
It's very possible that your computer is working fine and your monitor is your only problem.
Verify that your PC has fully power cycled. In other words, make sure your computer has completely reset - make sure that it's coming on from a completely powered-off state.
Often times a computer will appear to "not be on" when actually it's just having problems resuming from either the Standby/Sleep or Hibernate power saving mode in Windows.
Note: You can power off your computer completely while in a power saving mode by holding the power button down for 3 to 5 seconds. After the power is completely off, turn on your PC and test to see if it will boot normally.
If you don't resolve the problem by troubleshooting to the specific beep code, you can always return here and continue with the steps below.
Important: If clearing the CMOS does fix your problem, make sure any changes you make in BIOS are completed one at a time so if the problem returns, you'll know which change caused your issue.
Verify that the power supply voltage switch is set correctly. If the input voltage for the power supply is not correct (based on your country) then your computer may not turn on completely.
There's a good possibility that your PC wouldn't power on at all if this switch is wrong but an incorrect power supply voltage might also prevent your computer from starting properly in this way too.
Reseat everything possible inside your PC. Reseating will reestablish the various connections inside your computer and is very often a "magic" fix to problems like this one.
Try reseating the following and then see if your computer begins to display something on screen:
Note: Unplug and reattach your keyboard and mouse as well. There isn't a great possibility that the keyboard or mouse is causing your computer to not turn on fully but we might as well reconnect them while we're reseating everything else.
Reseat the CPU only if you suspect that it might have come loose or might not have been installed properly.
Note: I call this out separately only because the chance of a CPU coming loose is very slim and because installing one is a sensitive task. This isn't a big concern if you're careful so don't worry!
Check for causes of electrical shorts inside your computer. This is often the cause of the problem when the computer powers off by itself but certain shorts can also prevent your computer from booting fully or showing anything on the monitor.
Test your power supply. Just because your computer's fans and lights are working does not mean that the power supply is functioning properly. The PSU tends to cause more problems than any other hardware and is often the cause of a computer not coming on all the way.
Replace your power supply immediately if it fails any test you perform.
Important: I want to make this point very clear - do not skip a test of your power supply thinking that your problem can't be the PSU because "things are getting power." Power supplies can work in varying degrees and one that isn't fully functional needs to be replaced.
Start your computer with essential hardware only. The purpose here is to remove as much hardware as possible while still maintaining your PC's ability to power on.
- If your computer starts normally with only essential hardware installed, proceed to Step 11.
- If your computer still isn't displaying anything on your monitor, proceed to Step 12.
Important: This step is easy enough for a novice to complete, takes no special tools, and could provide you with a lot of valuable information. This isn't a step to skip if, after all the steps above, your computer is still not turning on completely.
Reinstall each piece of hardware that you removed in Step 10, one piece at a time, testing after each installation.
Since your computer powered on with only the essential hardware installed, those components must working properly. This means that one of the hardware components you removed is causing your PC to not turn on properly. By installing each device back into your PC and testing each time, you'll eventually find the hardware that caused your problem.
Replace the nonworking hardware once you've identified it. These Hardware Installation Videos should come in handy as you're reinstalling your hardware.
Test your computer's hardware using a Power On Self Test card. If your PC still isn't displaying information on your monitor with nothing but essential computer hardware installed, a POST card will help identify which piece of remaining hardware is causing your computer to not come on completely.
If you don't have and are unwilling to purchase a POST card, skip to Step 13.
Replace each piece of essential hardware in your computer with an identical or equivalent spare piece of hardware (that you know is working), one component at a time, to determine which piece of hardware is causing your computer to not come on all the way. Test after each hardware replacement to determine which component is faulty.
Note: The average computer user doesn't have a collection of working spare computer parts at home or work. If you don't either, my advice is to revisit Step 12. A POST card is inexpensive and is a more reasonable approach than stocking spare computer parts.
Unfortunately, if you don't have a POST card or spare parts to swap in and out, you're left not knowing which piece of your essential PC hardware is faulty. In these cases you have little option than to rely on the help of individuals or companies that do have these resources.
Note: See the last tip below for information on getting more help via the PC Support forums on this site.
Are you troubleshooting this issue on a computer that you've just built? If so, triple check your configuration! There is a very, very good chance that your computer is not booting up completely due to a misconfiguration and not an actual hardware failure or other problem.
Did I miss a troubleshooting step that helped you (or might help someone else) fix a computer that's not showing anything on screen? Let me know and I'd be happy to include the information here.
Is your computer still not showing anything on the monitor? Let a community of computer support enthusiasts help out! Post the details of your problem in the PC Support Forum. Be sure to tell us what you've already done to try to fix the problem.