OK, very strange! Sounds like the system was very rarely used if at all..
In some ways that helps narrows down is issues.
Let’s check the onboard diagnostic LED’s what are they telling you?
[image|2246868]
'''LED 1''' - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.
'''LED 2''' - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
'''LED 3''' - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
'''LED 4''' - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.
=== Beyond checking the diagnostics! ===
First thing I would do is to replace the PRAM battery as it’s likely dead being unplugged from AC for more than six months would have drained it.
Then the question is what needs replacing beyond the PRAM battery is dependent on what the diagnostics tells us after we replace it.