I haven't got a problem so much as I'd like to understand how I have fixed one!
I turned the PC off as normal last night. When I went for a wee in the night I noticed that the green light at the front was flickering quickly so I tried, to no avail, to hold in the button to turn it off as the screen was blank. This morning I couldn't get it to boot up and noticed that the green power light at the back was also flashing and making a clicking noise.
Having researched it online the solution seems to be to fire a hairdryer into the back of the PC until the small green power light goes constant. I did this and it has worked.
Any ideas why please? I assumed that I need (or still may need a new power unit).
I had this fault on an HP Pavillion PC. The front power LED was flashing and the PC wouldn't switch on. There was also a small green LED at the back of the PSU which was flashing rapidly. I tried all manner of "fixes" from the Internet to no avail then a couple of days later I powered up the PC and it started first time. Unfortunately, after about a week the fault returned. I had read that this is the sign of a failing PSU so I fitted a new one. The PC has now been working for about a year with no further problems.
Ease of fitting a new PSU depends on the construction of the case. If the case is of the compact (horizontal) style then your choices are limited. However, if it is of a Tower style then so long as you go for one with the same, or greater, power rating then it's usually just a matter of taking note of the connections as you unplug them so you can connect the new PSU correctly. The PSU itself is usually secured by 4 screws from the rear.
If you need a more definitive answer then we'd need to know the model number of your PC.
As previously mentioned, it sounds something like dust, condensation or a pending faulty psu.
Changing a psu is a very easy task, if you have a similar or like for like replacement available. If you have a 300w psu, it might pay to upgrade to an higher wattage, because you will then have a 'bit to spare' which is only going to cost you slightly more on purchasing.
Remember to unplug from the mains before delving into the case. And don't let anything wonder in the case, that might effect other components!.
Maplin UK sell a range of power supply units. I replaced a defective one in a Compaq Presario desktop PC. It was a 300w psu and cost 19.99 pounds. Part number WF87U . Easy to fit just photo and label all connectors first. Then replace like for like.
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