Diagnosis
The microphone has been manually muted, either via the remote button or a voice command. The solid red light is the explicit mute indicator.
How to Fix It
Unmute it immediately. Press the microphone mute button on the top of your Fire TV remote (the one with a microphone icon and a slash). You should hear a chime and the red light will turn off. Alternatively, you can say "Alexa, unmute yourself."
If That Didn't Work
If the remote button doesn't work, do a physical check. Look for the physical microphone mute switch on the back left side of the Cube itself—ensure it's not slid to the mute (red) position.
Heads Up
A solid red light also appears during a factory reset. If you didn't mute it and it's red after a reboot, wait a few minutes—it may be in the middle of the reset process.