The indicator light on a switching power supply may remain off for the following reasons:

1. Power not connected: Check that power is properly connected to the input of the switching power supply and that the incoming power is normal. If power is not connected or the connection is poor, the indicator cannot light up.

2. Abnormal input voltage: The input voltage of the switching power supply may be below or above its rated range. Check whether the input voltage is within the specified range. If the input voltage is abnormal, the switching power supply may not operate properly and the indicator may remain off.

3. Switching power supply failure: The switching power supply itself may have a fault, such as a damaged switching device, a control circuit fault, or another component failure. These faults may prevent the indicator from lighting and the power supply from operating properly.

4. Indicator failure: The indicator itself may have a fault, such as a burned-out bulb or a lighting circuit fault. Check whether the indicator is damaged and try replacing it to determine whether the fault is caused by the indicator itself.

5. Overload or overtemperature protection triggered: If the load on the switching power supply is excessive or the power supply itself overheats, the protection circuit may trigger, stopping the power supply and leaving the indicator off. Check whether the load exceeds the rated power of the supply and ensure a suitable operating environment to prevent overheating.

6. Other faults: Other faults, such as a damaged circuit board, a disconnected wire, or another circuit fault, may also prevent the indicator from lighting. Check whether the internal circuits and connections of the power supply are normal and look for other potential causes.

If the indicator light on a switching power supply remains off, the following troubleshooting steps are recommended:

1. Ensure that power is connected correctly and check whether the input supply voltage is normal.

2. Check the switching power supply itself for faults, such as faults in the switching devices or control circuit.

3. Check whether the indicator is damaged; try replacing it.

4. Check whether an excessive load or high ambient temperature has triggered overload or overtemperature protection.

5. If necessary, use test instruments to check circuits and signals, or ask a professional to diagnose and repair the fault.

Please note that switching power supplies involve high voltages and currents. If you do not have the relevant electronics knowledge and skills, seek professional help to diagnose and repair faults.