If Google Voice Match has stopped working on your phone, here is a possible fix for you to try:
First thing to check is if you get a blank screen when you open Settings > Security & location > Smart Lock.
If you do, the screen lock method that you are using (PIN or pattern etc) is not allowing
- Go to Settings > Security & location.
- Scroll down to the bottom of the screen and tap Advanced to reveal additional options.
- Tap Trust agents
- Slide Smart Lock (Google) to off.
- Go to Settings > Security & location and tap Smart lock. You will need to unlock your phone to see the contents of this screen.
- Select Voice Match.
- Train the voice model by following the prompts.
Now, try unlocking the voice using voice again. If this did not work, try the following steps. Note that you will lose all Google Assistant related settings and preferences and will need to set it up again.
- Go to Settings > Apps > Google App > Storage > Clear Cache
- Tap on Manage Space > Clear All Data.
- Restart your phone and set up Google Assistant again.
Did this work for you? Let me know in comments!