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"LCD Backlight not working"에 답변Which OS are you running? Many newer OS’s such as Mojave on your unsupported iMac will disable the backlight. If that’s the case check out macforums.com unsupported Mojave thread for the fix. They have a link to the fix.
"My iMac will not not boot from or to ANYTHING"에 답변Have you reset SMC and PRAM? This model has a known defective GPU also. @dan ‘s request to attach an external monitor will tell you if this is the culprit. Inside the iMac there are 4 diagnostic LEDs. Before you go that far start it in safe mode and see what the verbose readout is. It may give you your needed information.