I'm going to go on the assumption that you removed the motherboard in order to replace the USB-C connector, therefore, you had to disconnect the LCD and digitizer cables.
The fact that the touch screen still works says you got the digitizer cable in right, but chances are the LCD cable (pictured below) is probably not quite in there correctly.
At this point, I'd suggest you open it back up and reseat the LCD connector. Follow the motherboard replacement instructions up to Step 25. Clean the LCD flex cable with isopropyl alcohol and reinsert it, making sure it's all the way into the connector before flipping down the retaining latch.
Nintendo Switch Motherboard Replacement - iFixit Repair Guide
Those wide flex cables can be a little tricky to get in just right, so make sure both of the little tabs on either side are butted up against the connector then lock it down. Hopefully that's all it is and you'll be back to gaming in no time!
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