Some players have found that the Daily Calendar has reset to Day 1 for them.
But don't worry! You will continue to receive daily rewards the same as before. The only difference is the number of days in a row your calendar will display.
There are a number of reasons this reset might happen: device updates, the time you logged in, and one-time glitches may cause this. Though your Daily Calendar progress cannot be restored, rest assured that your actual rewards are unaffected. You can keep collecting and enjoying your rewards every day!