This could be because you upgraded your operating system. In this case, please reinstall Google Text-to-speech. You can find and reinstall Google Text-to-Speech via the Google Play Store.
After installing you need to activate the app in the settings (Profile (Head Icon) -> Settings (Cogwheels Icon) -> Navigation). It might also be necessary to log out of the app, and you should be able to use your voice navigation again.
Wrong language in voice navigation