Closing down completely and leaving black screen

Imac closes down with a black sign saying:

"You need to restart your computer. Hold down the power button for several seconds or press restart button" OK but it still closes down.

Remedy: Place a horisontal blower under the left edge of the iMac

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Even worse now: When opening a USB-connected scanner leads to:

A sound like a smal spark inside the computer and a complete black screen.

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Assuming that the power supply in the computeres DEAD !!!!

Am I right or ???