Final answer:
The first action a manager should take after discovering water supply was shut off is to identify the problem, determining its cause and impact.
Step-by-step explanation:
When a manager finds that the water supply was shut off overnight, the first action they must take is to identify the problem. This includes determining the cause of the shut-off and assessing whether it's a local issue or a larger, systemic problem. It's critical to quickly understand the extent and impact of the water outage to plan for appropriate measures, which may range from notifying the relevant utility companies to setting up alternative sources of water if the outage is going to last for a significant time.