What is the root cause if a multimeter shows zero voltage on a working power supply?

If a multimeter shows zero voltage on the output terminals of a power supply that is known to be working (i.e., the input voltage is present and the power supply itself is not internally faulty), the root cause is most likely a tripped overcurrent protection circuit or a blown fuse within the power supply itself. Power supplies are designed with safety mechanisms to prevent damage from excessive current draw. If the load connected to the power supply draws more current than it is rated for, the overcurrent protection circuit will trip, effectively shutting down the output voltage to prevent damage. Similarly, a fuse is a sacrificial component designed to blow and break the circuit if the current exceeds a safe level. Resetting the overcurrent protection (if applicable) or replacing the blown fuse will restore the voltage output, assuming the overload condition has been resolved.