screen won't turn on after screen replacement

I attempted to fix my Galaxy S21 Ultra a few months ago (back in May) by replacing the screen. At the time, I couldn’t get the antenna out of the old screen, so I left the parts in a clean box until I could finish the repair.

Recently, I managed to get the antenna out and completed the repair. When I powered the phone on, the logo appeared twice, but it wouldn't turn on again. I’ve tried charging it, but there’s no response—no charging icon or signs of life.

Any advice on what might be wrong or what I should check next would be much appreciated!

I appreciate any help you can provide.