Diagnosis
The automation or "Scene" that links the sensor to the bulb has likely become corrupted or disabled. This is a common glitch after app or hub firmware updates, where automations silently break.
How to Fix It
Open the Aqara app and go to the Automation (or "Scene") tab. Find the automation that's supposed to trigger the bulb. Don't just look at it—open it to edit. Check that the condition is still "Motion Detected" and the action is "Turn On" your specific bulb. Tap through and re-save it, even if the settings look correct. This often re-establishes the internal link. After saving, test the sensor immediately.
If That Didn't Work
The issue might be with the bulb's connection, not the sensor. If the bulb is a third-party brand (like Philips Hue or Wi-Fi based), the link between the Aqara hub and that bulb's ecosystem (via HomeKit or another integration) might be down. Check if you can control the bulb directly from its own app or from the Apple Home app.
Heads Up
If you're using the Aqara app's "Automation" for this, note that these only run when the hub is online. If you're using Apple HomeKit automations instead, those run on your Home Hub (Apple TV, HomePod), and a problem there would also break the trigger. Know where your automation logic lives.