It sounds obvious, but sometimes you can flip the switch off without even thinking about it, and all of a sudden you have a problem.
Furthermore, make sure that all of your Hue bulbs are somewhat close together so that they can all receive a signal from your Hue Bridge.
This is what happened to me before I finally figured out what was going on.
I didn’t really like how all my Philips Hue lights were showing up in the Connect Sense app, so I removed the Hue Bridge from the app, but doing so completely unpaired the Bridge from Home Kit, resulting in Siri no longer working until I re-paired it within the Philips Hue app.
Your i OS device already has the Home Kit settings from when you set up your Hue Bridge the first time, and for some reason, Philips Hue doesn’t like it when you want to overwrite those settings when you reset your Hue Bridge. Go through the Siri setup process again in the Philips Hue app and it should work this time around.
If you set Siri up successfully within Philips Hue, but the Bridge unpairs randomly from Home Kit at some point, it’s likely because you removed the Hue Bridge from within another Home Kit-supported app.
The old globes sitting next to the new ones on the counter really show just how dingy they’ve gotten over the years.