How to turn Siri on
If it’s not working at all, Siri might be switched off. In iOS, go to Settings > Siri (in earlier versions of iOS you had to select Settings > General > Siri) and make sure the top slider, labelled Siri, is on, and green.
On Mac, open System Preferences and click Siri (on the fourth row). Make sure there’s a tick next to Enable Siri on the lefthand side.
Does your device support untethered Hey Siri?
Only the following devices support Hey Siri without being connected to a power source:
iPhone 6s Plus
12.9″ iPad Pro
-Reboot your device.
-Is Low Power Mode turned on?.
-Try recalibrating Siri.
-Restore your iOS device