i had this big problem after HDD replacement, and i'm going to tell what happened.
I replace the HDD and installed the 10.13.2, everything worked fine, no fan noise, the system run smoothly, but i didn't apply the glass just in case i had to fix the hdd sensor.
So i shut down the mac in the night and removed the power cord to avoid any type of issue.
In the morning i returned on the machine, try to power on and when the mac power on i smelled a burnt scent, maybe some component, in the upper right part of the lcd, soon back the lcd.
I have backlight on the screen, if i connect to external it works, as second monitor but it works, there are no sign of burnt components, neither on the lcd ( inveter etc.) nor on the mainboard , so, what else can i check??
I'm desperate, the lcd make some kind of interferences but no chance to get it working again