Grey Screen

Purchased a 27inch iMac 6 weeks ago so I am no expert.
For the second time in the last couple of weeks the screen does not get past the grey and featureless stat-up screen.
No logo, no spinning widget. Nothing. Nada. Zilch.
I can only get a reaction from the machine by pressing the on/off switch rapidly and then it restarts with the same effect. (last time this worked)
I unplugged the machine. No difference on replugging and pressing the on/off button.
Next, I unplugged the USB flash drives and repeated the same procedure. It then started up.
The wireless router was not connected, but a restart fixed that.

WHY?..............wasted half an hour on this today.