Unpaid invoices, liquidity failure and nerves in the company. How can I not lose my client and my money?

It starts innocently — the client is simply a fewer days late with payment. Nothing new, it can happen to anyone. But in time the invoices accumulate, the deadlines arrive, and in the account less and less funds. Then comes the stress, emotions and concerns about the future of cooperation. How do you recover your debts without burning bridges with a contractor?