Is it a sin to use a program that browses the Internet without a balance, after notifying the company and receiving no response?

If the company charges a fee for internet service and restricts its use unless there is a balance, then it is not permissible for you to benefit from the service for free, even if it is due to a system error. You must contact the officials to pay the company's due or obtain their permission and exoneration. However, if the company's custom is to provide the service for free, then there is no harm in you benefiting from it without a balance. The reference in this matter is the company's custom or its officials.