PC will not boot up

[sosman]

If you say you can hear the fans, that means it's not the power supply.
If nothing appears on the screen, that means it's not the disk either. If the disk was the cause, it would start showing numbers on the screen and then stop with the message that it can't find system disk or something similar.
If you hear multiple beeps when you turn on the pc, that means the it's the RAM (the memory) or the video card.
If you don't have any beeps, it it the CPU or the motherboard broken.
However, in some rare cases, it happens to be an imperfect contact on the motherboard that causes the problem, especially if you say this has happened before.

What I recommend is the following:
Remove the power cable, take the cover off and gently push all plugs on the motherboard and the RAM in the slots, making sure it is properly connected. Make sure you TOUCH THE METAL FRAME OF THE PC before touching anything inside to prevent an electrostatic discharge damaging your pc.
Sometimes excessive dust can create conductivity between two parts that should be separated or go inside contacts and degrade connections. if you see excessive dust, either blow it with compressed air or a pump or try cleaning it with the hoover (more tricky). However, remember - no touching any parts with the hoover hose and NO WIPING THE DUST with anything.

Edit:
One more remote posibility (considering you hear the clicks) is that the power supply is not powerfull enough to drive all the stuff and the disk is turning on and off. I've seen this happening when new components were installed of if the power supply was at the limit. In time, power supplies decrease in max output power. If this is the case, unplugging a few of the devices connected (like the CDROM or even the disk in the first stage) would see the pc come to life. This scenario however is not often encountered.

yeah take that!.... what he said.....lol

[Mom]

If you say you can hear the fans, that means it's not the power supply.
Edit:
One more remote posibility (considering you hear the clicks) is that the power supply is not powerfull enough to drive all the stuff and the disk is turning on and off. I've seen this happening when new components were installed of if the power supply was at the limit. In time, power supplies decrease in max output power. If this is the case, unplugging a few of the devices connected (like the CDROM or even the disk in the first stage) would see the pc come to life. This scenario however is not often encountered.

Well now, the PC is home and operating once again. We are $160 odd poorer. The power supply has been replaced. Your edit really interests me. We replaced the power supply on this PC less than 2 years ago, then the thing simply refused to turn on at all. This time however it was actually running and went into sleep mode overnight. We did not turn it off. In the morning the fans were running, but nothing would wake the thing from sleep. I turned it off and then powered it up again and the fans started ok but the box just made a clicking noise, would not fire up the PC proper.

I went to pick it up today and got the boss man. He decided to re-test the faulty power supply on a dummy in the shop. It started it no worries! Hmmmmmm, he thinks they may have missdiagnosed the problem so attempts to start our box with the old power supply, the fans start, but nothing fires up.

So, can you attempt to tell me in really simple English, why the old power supply will start a dummy in his shop and not our PC?

[Ixion]

So, can you attempt to tell me in really simple English, why the old power supply will start a dummy in his shop and not our PC?

Well, sort of, like fit a moped motor into a Goldwing and see how far you get. Then fit the same motor into the moped.

[steve_t]

How many watts was the old PSU (power supply) rated for and how many is the new one?

[centaurus]

Ixion is right on the money. My guess is that the dummy pc he tested the power supply on, needs less power than your pc (older pc/less components installed, etc...).