A few weeks ago I accidently pulled the mains lead out of the back of the power supply and ever since, when I press the power switch, all I hear from the PC is a faint clicking sound from the power supply where the mains lead connects, nothing else happens. No lights come on, nothing moves, the monitor doesn't swtich on, nothing. I have replaced the power supply and the mains lead and still, nothing happens. I desperately need to know what is wrong with it. Can anyone help?
Thanks for your time, Oli.

Usually when this happens it is a result of damaged or incompatable hardware (can also be because of a shortage of power)

Try removing an item which isnt needed for startup (a secondary hardrive or secondary cd drive) and then try booting it up again.

If that still doesn't work I have no idea, as a power outage should not damage your motherboard etc... in anyway as the powersupply should protect it from this?

Check that you have the power supply connected up properly.

I am slightly confused as to why the monitor won't even switch on. When you mean switch on do you mean it won't actually turn on or it won't display anything. If you mean it won't switch on this is odd as normally monitors are connected directly to the mains unless you're using a kettle lead pass through type thing from the power supply, which I've only seen once.

The two things I suggest is check that the power outlets are actually working (as the monitor not working is weird) and check the power supply is connected up properly with only the minimum connected to it (motherboard, cpu, heatsink/fan and one RAM stick).

Failing this i reckon your motherboard is dead, if the monitor went the same way then it's the result of a large power spike, power supplies can only regulate so much, get a surge protector.

Thanks for the help, i'll try all the things you've said, i'm pretty sure i've connected everything back up properly. The PC had been running fine for about 6 months before this happens so i'm hoping it's not but, you might be right, it probably is a large power spike :/