Diagnosing doorbell transformer
Replace the chime, the coil is toast from the shorted wires when you weren’t home.

Open circuit voltage with the circuit at rest being 19-20 is nothing to worry about.   (a doorbell is a momentary load device, the circuit is at rest 99.99% of the time.  I avoided other terms like "open" and "closed" to describe the circut but based on your comment still confused.)