Switch on my iMac

Suddenly my iMac Intel Core Duo first generation didn't switch on: the monitor is completely dead, the green light is off.

I guess there is a problem on the local power source or on the mainboard, but the Apple store man told me that old spare elements are no more available.

Do you have any suggestions?

I'm in dispear, please save me!!! D^;

Possibly a bad power supply. You can try to locate one on eBay or one of these two places may have it:

iFixit: The free repair manual

Mac Repair - Mac Parts and Service for Apple Macbook, iPhone, iPad

Many thanks for your information. Before to buy the power supply I need to avoid any doubts about the logic board.
Any ideas?

Well, since it's almost impossible to run diagnostics on the logic board without Apple doing it, and since replacing one is so expensive, I would take a chance and change out the PS if it's cheap enough to do so.