21.5" Late 2012 iMac | Display or GPU issue
Without putting hands on, it's impossible to know for sure if it's a GPU or an LCD issue. However, a screen that has a delineated bright area and a dead area will most likely be the display panel and not the GPU.
It's not clear what you mean by  a &quot;display pin issue,&quot; but if you're referencing an issue with the display LVDS cable, then no, it won't be related to a faulty cable.
If you have another Mac available, you can put this one into Target Disk Mode and use that Mac to boot off the iMac's internal drive.  You can then configure display mirroring and remote control (VNC) so that the next time you boot the iMac (normally), you'll be able to better diagnose it