DIAGNOSIS The far-field microphones are likely disabled. There's a physical switch on the side of the device that mutes the mics, and it's easy to bump accidentally.
FIX Check the left side of the device (when facing the Google logo). You'll see a small physical switch. If it's orange or showing an orange LED, the microphones are muted. Slide the switch toward the front of the device until the orange light turns off. You should hear an audible chime.
IF THAT DIDN'T WORK Background noise or placement can interfere. Move the Mini away from noisy appliances (fans, AC vents) and reflective surfaces like large windows. Ensure it's not tucked inside a bookshelf where sound is buffered.
HEADS UP If the mic switch is off and the device still can't hear you, test with a voice call from the Google Home app. If the caller can't hear you, the microphone array may have failed, requiring a hardware replacement.