1. There was an issue in the department to which the manager needed to focus direct attention. Besides hiring, the manager is responsible for tending to the needs of his/her department, and many times there are end-of-day or end-of week issues which divert attention from hiring.
2. While the position needs to be filled, there are other priorities within the organization and filling the position is not the primary focus at this time. Perhaps the department is working cohesively right now and there isn't a rush.
3. The manager may have left due to a family or personal emergency. It happens. Managers are people too!
4. The hiring person or group has not yet rendered a decision on who to hire. Perhaps they are still thinking about it, or perhaps they are checking references.
5. Maybe you are a second choice candidate and the first person has not decided whether they are accepting the position.
Regardless, stay calm and positive. It's also possible that the person simply forgot the time to call you. We've all said something in passing where the other person fixated on the specifics that we forgot. Give the person an extra 3-4 days, and then follow up with a call or email asking if there is any information you may provide to make the decision making process easier. Being helpful is always better than being needy and desperate!