From a power off state, does the imac chime when you power up? If so (hearing the chime), are you saying that you do NOT at any time see a light grey screen with a dark grey apple logo about center of the screen?
Do you have a recovery disk? Try booting off of the recovery disk and see if you get the display to work its normal routine. Also, be listening if the optical disk is being accessed during this time.
Find a flashlight and shine it onto the screen to see if the backlight just happened to fail on you.
If still dark on the screen, then re enter the imac to verify that all the cables are attached and plugged into their appropriate sockets
Let us know what you find out.
Terrence.
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== Update ==
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My apologies for misreading your post. Thanks for reviewing the facts again for me. As you know, when a system is working fine prior to an upgrade and fails in the aftermath of such upgrade then one can conclude that either the failure was only coincidental, OR that in the process of performing stated upgrade, that an unnoticed improper reassembly or damage occurred. I have observed that an lcd panel requires a data cable AND a cable to drive the lcd backlight. This was the reason i suggested shining a flashlight upon the screen to see if the panel was actually getting data. The reason for using an os x recovery disk was to ensure that display drivers was actually being loaded to drive the display and eliminating possible corruption or missing driver. But since you pointed out that the external DVI will drive an external monitor, I agree that booting from a recovery CD/DVD would not buy you anything. So, we are left with having to re examine the cables that were unplugged to remove / install the upgraded drive, looking for damaged cables or connectors OR cable to connector that was reversed upon reconnection. Good luck.
From a power off state, does the imac chime when you power up? If so (hearing the chime), are you saying that you do NOT at any time see a light grey screen with a dark grey apple logo about center of the screen?
Do you have a recovery disk? Try booting off of the recovery disk and see if you get the display to work its normal routine. Also, be listening if the optical disk is being accessed during this time.
Find a flashlight and shine it onto the screen to see if the backlight just happened to fail on you.
If still dark on the screen, then re enter the imac to verify that all the cables are attached and plugged into their appropriate sockets
Let us know what you find out.
Terrence.