Voice commands

I have been using the Talk Switch and Voice Commands successfully for a while, but just recently the function has failed. When I press the Talk Switch, the Voice Command overlay comes on to the screen, but none of my commands are recognized nor acted upon. In addition, the on-screen Shortcut Command buttons are dead and don't respond to touch. The blue buttons that control Voice Command features are active, so I can turn off voice prompts and the like, but none of my voice commands nor any of the green touch commands are heeded. Have I disabled the function in some way by mistake, and if so, by what means and how can it be restored?