If Cortana doesn't respond when you speak into your PC, this might be why:
Your microphone may be not be set up. In the search box on the taskbar, type set up a microphone, and then select Set up a microphone and follow the prompts.
You may not have a microphone. In the search box on the taskbar, type Manage audio devices and select the result. Go to the Recording tab of the box that appears. If you have a microphone, you'll see it listed here.