Overdue tickets are those that have not been resolved within the due by time specified by the SLA. When a ticket is overdue, it means that the promise the company made to the customer has been broken. This could be due to a lot of reasons - dependencies, miscommunication, frustrated customer, or just an overworked agent.
The first thing you should do when a ticket becomes overdue is mitigate the damage. The agent should apologize and explain to the customer the reason for the delay. Or the manager can jump in and get things moving. Either way, getting in touch with the customer and explaining the situation should be the first step.