My MacBook's Screen Not Working

Hi,

I tear down the MacBook to replace the thermal paste. I took a look at the iFixit Logic Board Replacement guide while doing this and I paid particular attention to the display socket but when I finished the process and turned on the computer, the screen did not work.

I can get an displaying with an external monitor, but the internal display is not working.

Why is this problem causing it? I suspect it's from the display socket, but I was careful this socket while I tear down this.