Sometimes another app is using the microphone and blocks the access to the microphone for other apps. Please close this app or if it is not clear which app blocks the microphone, close them all. If this does not help, reboot your phone.
In rare cases it has to do with the permissions. The ‘audio record’ permission is needed for the tuner. Maybe the permission has been revoked. There is
the possibility to withdraw permissions with Android 6 or higher. Please check: ‘Info’ -> ‘Permissions’ -> ‘Permissions’