If Cortana doesn't respond when you speak into your PC, try these steps to resolve the most common causes:
Check to make sure you have a working microphone. Check that your microphone is enabled and set as the default device.
Check your privacy settings. Your privacy settings can prevent Cortana from responding to your voice.
Check that "Cortana" mode is turned on. Note: The wake word is currently disabled in recent versions of Cortana but will be re-enabled soon.
If you're going through the set up experience, there may not be an issue with your computer or microphone.
For step-by-step solutions to each of these potential causes, see the "Cortana can't hear me" support topic.