Please help, mac won't turn on, tried everything!

Yesterday I was on the internet using my iMac (i think it is the early 2008 intel model)
The mac completely turned off on its own, and when I tried to turn it back on, it didn't work.
The screen is completely black, no grey or blue screen or any kind of light coming from it, the same as when its turned off.
I could hear it whirring as if it had started up, I'm not sure if this is the sound of the fan or not. There is no chime and no LED light.
I am completely sure it is plugged in at both ends, i have plugged and unplugged the power cable multiple times but still nothing.
I tried resetting the SMC (i.e holding power while plugging in powerb cable then pressing power button) - loads of dust seemed to blow out, and the fan was whirring alot faster/louder, but nothing changed after that.
I also tried holding cmd-option-p-r after pressing the power button, but that didn't make any difference at all. (PRAM reset)
Loads of important data is on this computer, I really need to get it working again
I do tend to leave the mac on for long periods of time, so it may be to do with this.
Thanks
Jethro

This sounds like a hardware problem. Take it to your local Apple Service Centre and ask them to give you a quote to fix it. If it's the screen or motherboard on a 3+ year old iMac it probably isn't worth the fix as the parts will cost a lot of money. In that case you can ask them to put your drive in an external enclosure and either access it from another computer or migrate it into the new Mac.

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Thanks in advance! :-)

is anyone there