Screen doesn't come on and no start up bong

@jcary start off with a few basic things like a SMC and PRAM reset:

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac (27-inch, Mid 2011):

1. From the Apple () menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off).

2. Unplug the computer’s power cord.

3. Wait 15 seconds.

4. Attach the computer’s power cord, making sure power button is not being pressed. Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.

5. Wait 5 seconds, then press the power button to turn on the computer.

Resetting Parameter RAM (PRAM)

PRAM stores certain system and device settings in a location that OS X can access quickly. Exactly which ettings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on keyboard: Command, Option (Alt), P, and R. You will need to hold these keys down simultaneously in Step 4.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until the computer restarts, and you hear the startup chime a second time.

6. Release keys.

After that check the diagnostic LEDs

The iMac (27-inch, Mid 2011) computer has four built-in diagnostic LEDs on the main logic board that can help you to troubleshoot the computer. The LEDs are located above the speaker connectors and slightly to the left. Here is a close up graphic of the troubleshooting LEDs.

LED #1

• Indicates that the trickle voltage from power supply has been detected by 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 computer has been correctly shutdown, the 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 power cord is kept connected and AC power source is present. Disconnecting the power cord and plugging it back will turn back this LED ON , even if computer is still off.

• After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power ource is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty.

LED #2

• Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) and power supply and voltage regulators are working correctly.

LED #3

• Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement.

LED #4

• Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED#4 is ON and there is no image on display, then the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement.

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• First disconnect the power cord from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since

• Verify AC source

• Verify known-good power cord is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that the MXM video card is seated properly

• Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, reseat or check backup battery if necessary for proper boot up):

If POST boot chime is not heard, go to No Startup symptom flow,

If POST boot chime is heard, go to No Video symptom flow.

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify internal DisplayPort cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

If external display works then verify/replace the LED backlight board

If external display works then verify/replace the LCD panel

If external display does not work verify/replace the logic board

Let us know what you find out.