It's not unusual (though not exactly healthy for the relationship) for spouses to not speak to each other when upset. It can last a day, a week, a month or more and does NOT mean they are on the "rocks". Some people need time alone, inside their own head, to work out their feelings. I personally think it is better to talk out problems ASAP, but not everyone is like that, some people need time to ruminate.
Don't panic if it happens again, and don't interfere, that is the last thing a parent wants. They are all grown up and will work it out how they choose to do so.