How to deal with a client who doesn't pay and doesn't respond to messages

Document everything in writing from day one - every agreement, every delivery, every attempt to contact - because without proof you've got nothing if you end up in court. Before you waste energy, send a formal message (not casual) with a payment deadline and make sure you have a record of it; a lot of clients shape up when they see you mean business. If that deadline passes with no response, talk to a lawyer about how to proceed legally in your area, because at a certain point chasing the guy down costs you more than what he owes you.