A checklist of sorts.
I get so many variations on this question so often that I just have to address it.
In short, that's nowhere near enough information for me to even guess.
Let me run through a list of some things to check.
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If your computer doesn't work, things to check include:
- Is it plugged in?
- The power supply
- The motherboard
- The computer's RAM
- The hard disk
- Expansion cards
- The video interface
- The video cable
- The monitor itself
- Probably more...
There are many things that could be wrong. A technician might be your best solution.
A checklist of sorts
Is it plugged in? The description matches a computer that's not plugged in at all. I've heard back from some very embarrassed people when I've suggested they check the power plug.
The power supply. This device, which converts wall power to the internal power used by computer components, could be broken or failing. It's usually located inside your computer right behind the power connection. This is a possibility even if the fan comes on or some lights light up.
The motherboard. All the primary computer components are connected to the motherboard, which could have a problem. It's rare, but motherboards have many components that are vital to the computer's operation.
The computer's RAM could fail or become loose in its socket. With a severe enough memory problem, a computer will typically not boot at all, but lights might come on and fans might run. (An intermittent memory issue is more likely to cause crashes after the computer is running.)
The hard disk could have failed. Typically you'll see some activity, but there are failure modes that result in a very dead-looking machine.
Any expansion card could fail in ways that could cause the entire computer to become inoperable.
The video interface could be dead. If your computer's doing something but you can't see it, how do you know if it's functioning or not?
The video cable could be unplugged or broken. Same idea: if you can't see it, is it really there?
The monitor itself could be unplugged or broken. Still the same idea. Lights might blink, disks might whir, and your computer might be working... or not.
I could go on.
Complex devices have complicated failures
Hopefully, by now you get the idea. If the computer fails to boot with no clues, no lights, no nothing, or even if the fan does run and some lights blink but you still see nothing on the screen, it could be anything.
There are so many things that could be wrong, there's just no way to know, with so little information, exactly what might be broken.
My recommendation is to take the computer to a technician who can diagnose where the problem lies.
There are steps you can try yourself to further diagnose the problem, depending on your level of comfort and expertise. Replacing components or removing optional components to see if anything makes a difference is one typical way to start.
After double-checking that it's plugged in, of course. 
Do this
Whenever you ask for help, gather information for whomever you're asking. Reviewing What Information Should I Provide When Asking for Help? is a good start, as it covers much of the information that could be valuable to and appreciated by whomever you reach out to.
Typically, those who ask this question without providing details will probably be better served by having someone more knowledgeable look at their machine.
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I had a PSU die on me. In this case, it was a quick diagnostic : There was a low but sharp noise coming from it followed by an ozone smell drifting out as the computer shut down suddenly.
So, case #2.
I also saw an HDD with the power connector plugged the wrong way and another IDE drive with the ribbon connected the wrong way. Both dead.
I’ve also seen a few drives where the boot sector went bad.
And if it’s on a power strip, make sure that’s turned on as well. That bit me last month, but thankfully an easy fix.
Back when I worked as a technical Support Agent for an Internet Service provider, I got a few calls from time to time where the user complained that their computer wouldn’t start after having our Internet service ‘installed’, so I’d ask them “Do you see anything on the screen?” then “Are there any lights on the hard drive box?”, then I’d ask “Can you check that the computer and monitor are both plugged in?”. As often as not, after they made those checks, they’d sheepishly tell me “It’s working now, it must have got unplugged”. If it was plugged in, I’d have them check that the video cable from the monitor to the computer ‘box’ was securely connected at both ends. If that didn’t solve the issue, I’d check that a lamp or other device could be connected to the same outlet, and work correctly. If so, I’d direct them to their computer seller/manufacturer for support, otherwise I’d tell them to check the circuit breakers in their home. One time, after going through the above list of questions, the user informed me that “The lights are out all over the neighborhood” when I suggested the circuit breaker may have popped, so I told that customer “Call back if the computer doesn’t start after the power comes back on”.
As an Internet Service Technical Support Agent, their computer not starting was outside the range of things I could offer help with, but I always wanted to eliminate the easy possibilities/solutions before sending them to support options that would probably have a cost associated with them.
These are a few of my experiences as a Tech-Support agent,
Ernie (Oldster)
I’d have to agree. # 1. Is it plugged in? In my experience as a Sr. Field Tech, the first thing I would ask… well, you know, Leo.
9 times out of 10, it wasn’t plugged in..
Just my 2 cents worth…
Leo, you mention making sure the computer’s plugged in. I’d add that both ends of the power cable should be checked, then test the power outlet with a lamp or something similar, considering that the provided information includes the fact that “. . . the light to the hard drive is not on”. At least, that’s where I’d start,
Ernie (Oldster)
Hello, I wanted to add that there are some power supplies with a on/off toggle switch on them. It can accidentally get turned off when cleaning house, connecting/disconnecting cables or moving the tower.