Why does the MacBook Pro doesn't boot up? (after top case replacement)

Hi!

I have installed a new topcase on my Macbook Pro (a1278) mid 2012.

Before installing it, the computer worked fine without a keyboard (I would usually remove the battery and power it on with the magsafe cable, since the keyboard wasn’t working).

Then I replaced the topcase (with Keyboard, no trackpad) and now the computer doesn’t turn on.

I tried powering it on with the magsafe and even with the Logicboard’s power pads and nothing.

I have attached a picture in case anyone has any suggestions?

Thank you!