Hi Henry,
That's an odd one, since the normal procedure for replacing a screen doesn't involve the front camera at all. Is this a screen replacement you did yourself, or did you have a shop do it?
If it was a shop, you would need to take it back to them and tell them they need to fix whatever they messed up.
Otherwise you'll probably need to open the phone back up and check the front camera. I've seen a couple of cases where there was a piece of film over the camera opening that was meant to protect the glass but needed to be removed before assembling the phone and wasn't.
That's the most likely cause, but if that's not it, you'll probably have to check the flex cable for the front camera and make sure it didn't get damaged durning the screen replacement. Unplug it and check the connector on both the logic board and the flex cable. Clean them with 90% or higher concentration of isopropyl alcohol, then reconnect the camera, making sure it's plugged in fully and firmly.
Check that camera opening for any obstruction and if that's not it, check the camera connection and let us know what you find.