A zero SleepScore means that the sonar technology was paused through the entirety of your night. This is due to your smartphone playing audio from another source, such as directly from your phone speakers, through another app, or through a Bluetooth connected device, such as air pods or a speaker.
If you get a zero SleepScore in the morning, try to disconnect your smartphone from any Bluetooth devices before bedtime. You can also check to make sure there are no background apps playing audio prior to tracking.