Have you tried carefully pressing along the display module margins? Display module and the rest of the phone are held together by screws and plastic clips, and the latter sometimes get loose from shocks like a drop. You will hear the clips clicking back into place when you press the margins if this is the case here.
I also seem to remember a previous topic in this forum where someone described a trick s/he did with the contact areas (inside) between display and core module, but I cannot recall details right now.
And by the way, welcome to our community forum! 