iMac 27 "A1312 mid 2011 doesn't recognize a working GPU

Dear friends,

my iMac 27 "A1312 2011 has just stopped working and isn't showing any image. I turn it on, It chimes but the 3rd LED indicating the video is communicating with the logic board doesn't turn on. Initially, I thought it could be the video card but I tested it on my friend's iMac, and it was working flawlessly. Everything seems to be working fine, but the video doesn’t appear. I tried to reset the VRAM and SMC unsuccessfully. The %#*@ LED doesn’t light up. Can anyone give me a tip on how to work this out?

This iMac never let me down.

I'd appreciate any contribution that can be made.