Why no number is shown
Readiness is built from the sleep and heart figures your own wearable recorded overnight. When too little of that night arrived, no number is published, because a score standing on two hours of data would move for reasons that have nothing to do with you.
What to check first
Open your device app and let it finish syncing, then pull the Today screen down to refresh. Most missing scores are a sync that has not landed rather than data that is gone.
If the device has not synced for days, reconnect it under Settings, then Devices. A provider connection can expire on their side without telling us.
While your baseline is warming up
A new account spends its first nights learning your own normal. Until that window closes the card shows its progress instead of a score, and the day it completes is shown on the card.
A day you hid
A day you marked hidden shows no score by design. Unhide it from your history and the score returns unchanged.