Screen wont turn on after USB-C Port Replacement

From your description the iPad is working and powering on so the obvious culprit is the screen itself is no longer displaying. The most likely failure point is the ribbon cable that connects the screen to the logic board. If the connector isn't seated exactly right or if it got pinched in the cover that screws over the connectors or between any other pieces of the reassembly it can cause it to stop/intermittently working. Have you visually inspected the connector cable and checked the connection where it plugs into the main board? You can also try starting it up with the screen off and the cable connected but not screwed down, this way you can gently flex the cable and connector to see if the screen flickers or stops working in certain positions. If this does nothing the other options are the screen itself being damaged, under undue strain, or even bent and possibly the graphics chip on the logic board being damaged or affected in some way (these are far less likely but worth considering if nothing else works).

Hope this helps and best of luck!