Diagnosis
The bulb is in a software-locked or unresponsive state. It has likely lost its default 'power loss recovery' setting due to a firmware command or glitch.
How to Fix It
Open the Tapo app, select the bulb, and go to Device Settings. Find the 'Power Loss Recovery' or 'Status When Power Returns' setting. Change it from 'Last Status' or 'Custom' to 'Turn On'. Now, use the wall switch to cut and restore power; the bulb should illuminate.
If That Didn't Work
The bulb's internal relay may be stuck. Cut power at the switch for 60 seconds. Restore power. If it remains off, perform a factory reset via the switch (off/on three times) to clear all internal settings.
Heads Up
Using the physical switch frequently can corrupt Wi-Fi connectivity, as the bulb needs a steady power supply to maintain its connection. Use the app for regular control.