Here's the on-board Diagnostic LED info for reference. Can you give us a bit more on what happened with your system.
LED 1 - Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will turn on when you connect the iMac to a working AC power source. The LED will remain on as long as the computer is on or asleep.
When the computer has been shut down correctly, LED 1 behavior may differ:
If a startup event is scheduled in System Preferences/Energy Saver, LED 1 will stay on after a correct shutdown. If no startup event is scheduled in System Preferences/Energy Saver, LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the computer is still off.
After disconnecting and reconnecting the AC power source, this LED could remain off: If the AC power source is missing or disconnected.
If the logic board is disconnected from the power supply or the AC receptacle.
If the power supply board is faulty.
LED 2 - Indicates that the computer is turned on. This LED will be on as long as the computer is turned on (but is not asleep) and the power supply and voltage regulators are working correctly.
LED 3 - Indicates that the computer and 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, the backup battery (on back of logic board) may need to be reseated or the video card might be installed incorrectly or need replacement.
LED 4 - Indicates that the computer and LCD panel are communicating. This LED will be ON when the computer is turned on and a video signal is being generated. If LED 4 is on and there is no image on the display, the LCD panel or the cables between the LCD and logic board might be installed incorrectly or need replacement.
LED Startup Sequence
LED 1 = Power is available.
- If no LED is visible:
- Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status. Reconnect the power cord and check the LED status again.
- Verify the AC source.
- Verify that a known-good power cord is connected.
- Verify the cable connection between the AC inlet and the power supply. Verify the cable connection between the power supply and the logic board. Verify the power supply.
LED 1 + LED 2 = Power is available and the system is turned on. If the second LED is not visible when the power button is pressed:
- Verify that the power button is connected to the power supply.
- Verify power button functionality.
- Verify the cable connection between the power supply and the logic board. Verify the power supply.
- Verify the logic board.
LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.
- If the third LED is not visible after the system is turned on:
- Verify whether the boot chime is present and fans are running when turned on (reset the SMC and PRAM and verify backup battery voltage for proper startup).
- If the POST boot chime is not heard, go to the No Startup symptom flow. If the POST boot chime is heard, go to the No Video symptom flow.
LED 1 + LED 2 + LED 3 + LED 4 = Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.
- If the fourth LED is not visible after the system is turned on:
- Verify the internal DisplayPort cable (eDP) connections between the LCD panel and the logic board. Inspect the LCD display cables for cable damage.
- Verify external video functionality and proceed according to the result:
- If an external display works, verify/replace the LCD panel.
- If an external display does not work, verify/replace the logic board.