iMac Intel will not turn on, ideas?

[ProstheticHead]

So my intel iMac suddenly shut itself down tonight and is completely unresponsive when I press the power button. If I put my ear next to the back casing I can hear a high pitch noise coming from the computer while it is plugged in.

Any ideas as to what is going on / a fix?

[chrismacguy]

Try unplugging it, giving it a while and then turning it on again, also make sure all the vents are unobstructed, as it sounds like its heat related. That high-pitched noise sounds like it could also be a PSU issue (None of my Mac Desktops since my iMac G3 have hummed).

[goldenpiggy]

Also try pressing down the power button for a while instead of the usual press and release (keep pressing until you hopefully hear the familiar startup tone). It worked for me after my 27" i7 refused to powerup after a power failure.

GLTY...hope it's not the power supply.

[mike.coulter]

I'd recommend ringing Apple before you mess around with it.