Why is my iMac still running but the screen is off?

My iMac is running like applications and such will open but the lcd screen is black. I have put new thermal paste on the GPU, I have made sure all cable connections are tight, and I have even recalled the GPU solder. One thing I might think is causing it is that the GPU temperature cable is unplugged because its being a pain in the but to install. But I am not sure it is that because I have my fans running on high speed because HDD temp cable is unplugged because I recently upgraded to a SSD. Any answers are greatly appreciated.

Thanks