Black Screen After Replacement

Hi guys,

A couple of days ago I decided to replace the HDD with a SSD. In the process I accidentally cracked the screen. I ordered a replacement screen. While waiting for the replacement to come in, I hooked up the original cracked screen so that I can use it while waiting for the new one. All was well.

Today, I received the replacement screen and hooked it up. However, when I turn the iMac on, I get the startup sound but it's just a black screen. I hooked up the original thinking maybe it's defective and I get the same black screen. Also, the fan is running at full speed when I power it on. I've transferred the thermal sensor cable from the screen and that doesn't seem to have made a difference. Any insight? Thanks.