Siri is not working, but it used to work. What is wrong?

Go to Settings. Tap Siri & Search. Turn off Listen for "Hey Siri," then turn it back on. When the Set Up "Hey Siri" screen appears, tap Continue.

If "Hey Siri" isn't working on your iPhone or iPad - Apple Support

I, too, have the same problem. Turning it off and on again doesn’t solve the problem.

“Hey Siri” doesn’t work at all.

Holding the side button works, but there is no sound. Everything is set up same as before. Phone is not on silent, and at full volume.

Siri stopped working several days ago.

I have the same problem. Turning it off and then back on is documented in Apple Support at https://support.apple.com/en-us/HT207489. That does not solve the problem.