This is a feature that does not get backed up like your game progress and Gold bars do so once lost, you'll have to start again.
It can sometimes be reset with an update of either your device's operating system or the game itself. The calendar can also start over again if you change the date and time or switch the time zone on your device, or, after many technical issues that can occur in the game or on the device you play on.
The value of the rewards will increase after day 7 so even though you need to start over, you're still getting a free booster every day.
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