What do you do when a customer goes radio silence when their bill is due?

I would give some time out of respect and compassion. You never know what people are dealing with in their lives.

After about a week or so of no contact it may be time to knock on their door in the evening one day when you think they would be home. Start with being respectful and understanding that times could be tough, but also stern in that you would like to be paid for the work you have completed.