Replaced LCD/Digitizer now Mac won't turn on

MacBook Air "Core 2 Duo" 1.4 11" (Late '10)

Hi!

First time posting any where with a question!

So, I replaced the LCD of this Mac, initially by just changing the LCD, then by changing the entire assembly.

The computer booted fine with the broken LCD (besides the crack), but when I replaced the LCD, the computer's fan would spin for a few seconds then turn off. I figured it was the LCD, and decided to get a new assembly.

I replaced the assembly and the same issue persists: The fan will spin for a few seconds but nothing will happen.

What I have done:

I have followed all the instructions of this link.

Replaced the display assembly.

Placed the SDD in another a1370 Mac (worked fine).

Any suggestions on how to proceed? Is replacing the motherboard the only possible next option?