Press power dear. Behind the screen on the right.
Press it again!
Press it for a second or so!!!
Oh for gawds sake let me do it!!!
(Leaps out of bath).
oh you're right, it doesn't seem to be working.
Fortunately, my dead iMac might only be resting, as it may have succumbed to a common enough power supply fault. Having gone through the support wizard, and poked around inside in the approved manner, and come to naught, I've driven it down to a nearby repairer. This could be a long week ...
Update: it lives! And Flickr has gone to "Gamma". And Firefox pops another update!
All at once!