White screen after thermal paste changed

Hi there, My problem is this:

My MacBook Pro 15” Mid 2012 was fine everything worked normally.

Yesterday I changed out battery with a new one and I had an idea to change the thermal paste, I removed all screws as shown in tutorials, removed the logic board changed the thermal paste, assembled everything back to there places and powered on the system and here is where I’m stuck! Greeted with a white screen!

I hear the chime of powering on but nothing shown in the screen just a white screen

I removed the RAM , the SSD did all sort of resets, but nothing still a white screen

Any suggestion ???

Thanks in advance