Hello, and thank you for using this service.
Sorry to hear you are having trouble. I will help you now.
Is the problem only with the zero key? If so, do both zero keys fail (on the numeric pad and the keyboard)?
Open the Speech pane of System Preferences and check whether either the listening key or the speech key has become set to zero. if so, change the setting.
If this doesn't help, please reply for more troubleshooting.
Please let me know if that fixes the problem, or if not the results of checking speech settings. thanks.
Does the zero key fail on both the numeric keypad and the keyboard itself?