Siri not responding on iPhone

Toggle Siri off and back on in Settings. If in Settings you have "Hey Siri" as the activation word, then it will not respond with just "Siri".

If this only happens when your iPhone is locked, then make sure that setting is turned on at Settings > Apple Intelligence & Siri > Allow Siri When Locked.

If you are referring to Siri not responding by manual activation with the Side Button, then follow up with that information or any other that may be helpful to advise further.