What can I do if a customer has not paid me in over 10 months for a cleaning service?

We are a cleaning company that has cleaned a facility for about a year, but towards the end they did not pay for a few months and we ended up stopping the service because they were not compliant with payment. They owe us $900 for two invoices and they have been overdue for about 10 months. We try contacting them and they do not answer through emails or calls. We do not know what else to do anymore.