iMac failing to start

Black screen

Quietly whirring fan

No start up chimes

Yesterday there was a momentary power loss and my iMac went off (without my realising) and when I came back to it perhaps 3 hours later, there was a black screen which would not re wake when the mouse was given its usual jiggle.

Previous power losses have not caused any problems of this sort.

The iMac has been working impeccably for a couple of years, no problems - going to sleep and re waking when asked to.

After the power loss the fan was working quietly in the background. I turned the Mac off by pressing and holding the power button on the back. I tried the reset - unplugging power supply from back of Mac and waiting 30 seconds and then re starting a recommended in the Mac handbook - several times. Nothing changed upon re powering.

Black screen

Quietly whirring fan

No start up chimes

I have tried other recommended actions - eg the one in the relevant Mac manual - which is command and R upon powering up but it is my impression that the keyboard is not talking to the computer.

I am aware that this is similar to other questions asked by iFixit people but nothing seems to fit the bill quite and before I take it off to a repairer I wondered if anyone has a suggestions.

Thanks for your help