My iMac is dead. Zero response from the power button. Have had this a few times since the power cuts have started again. It's been through the hands of Denis (old repairs.com), who thought it was a temporary issue. Runs via UPS, have been okay for about 7-10 days and now dead in the water again. The moer in ... this is a work machine.
What now? Take to iStore? Insurance? How long will this take?
try this, can be tricky to do if you are not close to the power switch (mains)
with mac powered off, switch off power supply at wall socket (or disconnect power cable from mac.
while disconnected from power source, gold down mac's power button for about 15 seconds.
then without releasing power button, turn on power at wall socket (or connect power cable if you used that method to disconnect from power source)
keep holding power button down for a further 10 seconds, then release power button and immediately press it again to power up the mac.
see if that gets it to power on.
if it does, in all probability the fans will come on at full speed and there will be a loss of certain functions while powered up like this. if it powers up using this method, then the power management unit on the board had issues with the smc.
either way, submit an insurance claim for a new mac