iMac mid-2011 not booting, black screen

Hi all,

I need some help with an iMac mid-2011 (Intel Core i5 2400S, Radeon HD 6750M) that no longer boots. The issue started after I swapped the HDD for a SSD and installed Windows 10 with BootCamp. It worked perfectly for a few days, at some point I logged into macOS and rebooted, and after that it stopped booting.

Here are the symptoms, on startup:

• the optical drive makes the usual seek noise
• the chime can be heard
• nothing is displayed on the screen, it remains black but the backlight is active
• the four diagnostic leds on the mainboard are lit

And the things I tried:

• various key combinations (Command+R, Option+Command+R, Option+Shift+Command+R, Option+Command+P+R)
• reinstalling the old HDD
• unplugging the computer and removing the BIOS battery for one hour, then retrying
• swapping the RAM: there are 2x2GB (original modules) + 2x8GB, I tried 2x2GB only and 2x8GB only in different slots

Any help would be welcome. This iMac is a bit old but still fairly capable for Word/Excel and basic gaming and I would really like to repair it.

Thank you