Hi everyone,
Got a late 2012 iMac on Mojave which I always have plugged into an external display through miniDP to HDMI converter, one day it just decided to stop recognising the monitor, I tried the same cable and adapter with a 2015 macbook and it worked fine, tried shutting down, resetting NVRAM, SMC to no avail, I even tried a different monitor and it wouldn’t even see it (the old one it would see but the monitor goes to sleep, assuming because it was already saved)
Update: it seems the Thunderbolt ports are not functioning, system information showing no hardware was found under Thunderbolt
Not sure where to go from here
Since the NVRAM and SMC was unsuccessful… you are looking at replacing the MLB (main logic board) to get functioning Thunderbolt ports. Your late 2012 iMac is Vintage in Apple’s eyes currently. It will join the Obsolete list sometime after the first of the year. Not sure I can recommend you spending $600US to repair that model. You may be able to purchase the same model for the same money.