iMac Mid 2011 21.5" display stopped working

I recently installed a SSD in my Mid 2011 iMac 21.5" (2.5 Ghz Core i5, AMD Radeon HD 6750M 512MB, 12 GB of RAM, MacOS 10.12 Sierra) using the Other World Computing (OWC) interface cable that allows MacOS to monitor drive temperature and not treat it as an unsupported drive (https://eshop.macsales.com/item/OWC/DIDIMACHDD11/).

After I performed the change and buttoned the computer back up, the built-in display would not turn on and the fan was turning at max speed. I attached an external monitor (which MacOS automatically treated as the primary display) and verified the SSD is working fine and got MacOS installed on the SSD and running normally.

Next, I opened the computer back up and found that the DisplayPort cable had come detached. I managed to get it reattached and also removed and reattached the Display Power Cable because at the time something made me think it had something to do with the fan. While I was inside, I also detached and reattached the other two cables.

Now when I boot up, the fan is no longer going crazy, so that problem is solved. And it seems like the iMac recognizes the presence of the built-in display because now it treats my external monitor as the secondary screen. But the built-in display is still black. Because the external is now secondary, I can't log in.

So, I was hoping to narrow down where the issue may be. My guess would be the backlight power cable or the DisplayPort cable. I would assume the Display Power cable is fine because MacOS knows the display is there. There's also the vertical sync cable, but I'm not sure what that does.

I thought maybe OWC would be able to help since they offer to perform these upgrades for a fee, but they just sent me to their YouTube video showing install instructions, which I had already watched a bunch of times before I started this process.

Thanks,

Mark