DIAGNOSIS The far-field microphone array on the Beam has likely been disabled, either by a physical switch, a software setting, or a glitch.
FIX First, check the top of the Beam. There's a touch-sensitive mic mute button. If the little LED next to it is lit solid white, the mics are muted. Tap it once to unmute (the light goes off). If the light is off and it's still not working, go into the Sonos app, select your Beam, then go to Voice Services and toggle the specific assistant (Alexa or Google) off and back on.
IF THAT DIDN'T WORK A deeper software glitch might require clearing the voice service cache. Remove the voice service completely from the Sonos app (Settings > Voice Services > Remove), then reboot the Beam by unplugging it for 10 seconds. Add the voice service back from scratch, re-linking your account.
HEADS UP The mics automatically mute when you make a phone call through the Beam or use the Sonos Voice Control feature for music. They should unmute automatically after, but sometimes they don't. Always check that mute light first.