If your LINK BAR has 4 orange LED lights in the front, this means you have "muted" the microphone which is located on the top of the speaker.
To unmute the LINK BAR use the "switch" and the front LEDs will stop illuminating orange. Now you can use the hot-words to interact with the Google Assistant/Google Search (varies from country).

Privacy information:
We call our button the "PrivacySwitch" for the moments when you don't want the bar to listen for the hot words.
Google has high privacy rules, and one of them is not to share end-user data with any third-party apps.