If a cow isn't milked, milk can build up in her udder, causing discomfort and pressure. This pressure can eventually stop the cow from producing more milk. Depending on the situation, other issues can arise, including:
Normal drying off
If the cow is producing less than 5 kg/day when milking stops, there are no problems. This is the normal way of "drying off".
Uncomfortable pressure
If the cow has recently calved and is producing a lot of milk, the pressure could be uncomfortable and infection could occur.
Mastitis
There could be a risk she gets mastitis if she is suddenly stopped being milked.
Bruising, udder injury, sickness, and death
All this from Google AI, who has apparently lived on a farm and learned life's lessons the hard way.