Diagnosis
The digital chime (the little white plug-in box) isn't communicating with the doorbell. This is usually because it lost its connection during a Wi-Fi change or power outage, or it's plugged into a bad outlet.
How to Fix It
First, check the chime itself. The light should be solid white. If it's off or blinking, unplug it and plug it directly into a wall outlet (not a power strip). Wait for it to boot (solid white light). Then, in the Ring app, go to your Doorbell 4 device settings > Chime Settings > Digital Chime. Select "Reconnect to Chime" and follow the in-app instructions, which will involve pressing the button on the doorbell.
If That Didn't Work
If the chime has a solid light but won't reconnect, you need to perform a factory reset on the chime. Hold the button on the side of the chime for 15 seconds until the light flashes. Then set it up as a brand new device in the Ring app under Set Up a Device > Chime.
Heads Up
If you have an existing mechanical chime (the one that goes "ding-dong"), the Doorbell 4 requires the included white bypass plug to be installed inside that chime box to protect it. If you didn't install that, it could have damaged the mechanical chime, but your digital chime should still work independently.