Have just cleaned some dust from inside pc (desktop box), plugged everything accidently left socket on heard slight spark when pushing connector in. firstly mouse and keyboard wouldn't work, i turned it off and restarted now no screen moniter says no signal

Any ideas what i have done cheers mike

accidently left socket on

Do you mean power was still connected to the machine? ie; Machine wasn't unplugged?

heard slight spark when pushing connector in

Which "connector"?

mouse and keyboard wouldn't work, i turned it off and restarted now no screen moniter says no signal

These are indications of a bad motherboard. You could possibly have damaged the motherboard.

tower was unplugged but power lead still plugged in and on heard spark as pushed power lead back into tower

tower was unplugged

So NO power to the machine as it is unplugged.

but power lead still plugged in

Do you mean the bundled group of wires from the power supply (inside enclosure) to the motherboard main power connector? (20 or 24 pin)

heard spark as pushed power lead back into tower

This is the power cord from the house/room wall power outlet to the rear (outside) of the power supply?