What did I do wrong? (newbie)

**First time replacement**

WARNING THE FOLLOWING POST WILL MAKE YOU FACE PALM HARD / ANNOY YOU

I have a working Samsung Galaxy S4 with a lose/faulty port charger and a motherboard that is spacitc at times (e.g. rarely it would say no sim card was inserted when there was one , or just system would be wonky ((it could be a broken system tray XD LOL ))).

I had an S4 with a broken digitizer. I figured it would be a 'bright idea' to just take the parts out of the broken digitizer and put them into my working phone. After replacing said parts, when i went to turn it on, it turns on (light turns blue the bottom buttoms flash) but then nothing after that. I ensured everything clicked and was securely connected. Then i just replaced my normal parts back into the working regular phone and now it too faces the same problem.

Edit: i tried and i clicked on the home button and the buttons led flash on and there was a green rectangle on the side of the screen for like one second and then vanished. Still darkness and nothing comes on.

What did i do wrong? :'(