Black screen but phone is working

At the very least you're going to want to re-adhere the screen to the chassis. I don't see any iFixit adhesives for the A21 but they do sell some for the S21, perhaps this can be used? iFixit S21 adhesive

If you're willing to purchase it from someone else, I did find this (much more likely to work): External Supplier of A21 screen adhesive

Whats most likely is the connector needs to be removed and inserted again and perhaps cleaning of the inside. With this, check for water damage in the motherboard connector which may be fixable by further cleaning.

On the other hand, if the screen has sustained damage, its likely just a broken display. iFixit does not sell A21 parts but I did find this Replacement Screen on Amazon.

If it doesn't appear to be damage to the display or a bad connection (whether by water damage or other particles) try connecting it to an external display and check that if the display takes inputs. If it does, it could possibly the ambient light sensor or a broken screen. If its a bad ambient light sensor, try moving to a very dark room to determine if its just a very dim screen or if it is actually broken. If its a broken ambient light sensor, try replacing that, otherwise you likely need a screen replacement.