Method 1: Retrain the Voice Model
Open the Google app, on your device,
Go to Settings, then search, and then Voice.
Confirm the ‘Voice Match’ option is ON.
Tap on 'retrain voice model'.
Then say ‘Hello Google’.
Then google assistant again returns to your voice as usual.
Method 2: Reboot your Android Device
If still Google Assistant Not Working on your Android device, then try to reboot your Android Device.
Press and hold the ‘Power’ button on your device for 20-25 seconds.
Then a few options will highlight on your screen.
Then tap on the ‘Restart or Reboot’ option.
Read more methods Click on: Fix Google Assistant Not Working