I didn't see the answer to this anywhere online, including on Phillips' terrible support pages, so I thought I'd put it here for you--the person who googled for the answer.
Here's the problem: I woke one morning and the Hue bulbs stopped responding to any automations that had been set up as well as to any commands from Amazon Alexa or Apple's Siri or from within the Apple Home app. But, they still worked with the Phillips switches and from within the Phillips Hue app on my iPhone. My first step was to unplug and then plug in again the Hue bridge, but that didn't fix it. Eventually, I moved the bridge from where I had it in my kitchen (more central) to the office, directly connected to the 16-port switch for my network. That fixed it ... I thought.
A few weeks go by and I wake up to the same problem. I tried switching out Ethernet cables, restarting the hub, you name it. I was close to completely resetting the whole Hue system, but I have a lot of bulbs and switches and sensors. That would be a serious undertaking. I did some googling and while I couldn't find my exact situation, I saw someone mention that a router firmware update munged up the settings in the Hue bridge. I did notice that both times, the indicator light for the internet connection was blinking, not steady.
At this point, I thought I remembered that both nights when this happened (and this was clearly happening while we slept), the Eero router had updated its software. Maybe the router update was munging the Hue bridge's internet settings. But how do I fix that?
The Solution: Unplug the Hue bridge ... for longer. It turns out I wasn't unplugging it for long enough in my previous attempts, maybe 15 to 20 seconds. I guess when manufacturers tell you to unplug for at least 30 seconds, sometimes it's true. (Not that Phillips said that since there are no support articles to help with any of this.) So I unplugged the router for more than 30 seconds, the internet light came on when I plugged it in and all has been well since.
And I think Eero's software updates have stopped munging the Hue's settings because I've had a couple of updates since then and no problems after.
I hope this helps someone out there.
