Helping people with computers... one answer at a time.
Diagnosing a computer that won't boot can be very difficult. In particular, spinning disks and blinking lights don't mean as much as you might think.
I'm having problem starting up my computer. When I press the Power button, it lights up and everything inside it runs, but nothing appears on my monitor. I don't know what's the problem. I already cleaned it and had reset the CMOS, but nothing works.
Well, I'll put it this way: just because the lights are on, doesn't mean anyone's home.
I get this and similar questions often. You turn on your computer. It makes noise, you can see lights - perhaps even blinking lights - and assume that it must be running.
It might be.
Problem is - it might also not be running at all.
The phrase "everything inside it runs" is actually impossible to prove just by looking.
You might very well hear the fans all start to blow, disks might start to spin, and you might see some lights come on, but about the only thing that you can say by all of that happening is that the computer has power.
I'll put it another way; if you actually removed the CPU or the RAM (don't), the fans would blow, the disks would spin, and some lights would come on.
And the computer would have exactly zero chance of actually working.
So don't read too much into blinking lights, spinning disks, and running fans.
The good news is that the computer appears to have power. The bad news is that you simply can't assuming anything more.
One of the more underrated and useful diagnostic tools for problems of this sort is the beep.
You know, the single beep your computer makes after you turn it on?
That beep is generated by software; specifically, the BIOS installed in your computer.
If you hear the beep, that tells you that the CPU is working, the BIOS seems intact, and at least some RAM is working.
That's a start.
If you don't hear that beep at all ... well, then you have problems. One or more of the CPU, RAM, motherboard, or some other component isn't working. In fact, it could still even be the power supply.
If you hear more than one beep; if it sounds like your computer is beeping out a Morse code letter with multiple short and/or long beeps, then it's trying to tell you something.
It's trying to tell you that something isn't working.
The POST (or Power On Self Test) has detected a problem and is using a pattern of beeps to try and tell you that something is wrong. What beep codes mean unfortunately depends on the BIOS manufacturer. There's a fairly comprehensive list here.
Why beeps? Because the BIOS may have detected a problem before turning on its video display hardware and software. Or the failure might have been in the video display hardware. With no way to show you the problem, beep codes are the only way that BIOS has to talk to the outside world.
If the computer gives you that one "all's well" beep when it starts up and you still don't see anything on the monitor, then it's time for a very fundamental test of your own.
Check the monitor.
More specifically, check that it's turned on, that it's connected, and that the connections are good.
If everything looks OK, this is a great time to borrow a monitor and connect it as an external monitor to your laptop or temporarily swap out your desktop monitor. Borrow a cable, too; they can and do fail. If the borrowed equipment works, then you know that it's your monitor or cable that needs repair or replacement.
If it doesn't, then things point back to the computer itself. And unfortunately, while you might prioritize looking at the video card as the most likely suspect, pretty much anything from RAM problems to power supply problems could still be at fault.
Just like you sometimes need a mechanic to repair your car, sometimes you do need a technician to diagnose your computer. Not only does a good tech have the experience to deal with this type of problem, they also have the equipment that allows them to look at things the average consumer can't see, and a collection of replacement parts that they can use to quickly test individual components on your machine.
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