aCalendar does not save any calendar data, the app is just a user interface to the Android calendar storage.
If events have disappeared please check first: Events from one or more calendars missing (wrong profile selected)
If this is not the case please check following, depending of which kind of calendar you are using:
- For local Android calendar please see: Using local data
- For cloud calendars please check that your data is all fine online in your cloud service, after that please have a look at our General sync issues page. If you still have problems you may want to try to reset the Android calendar storage: Reset the Android calendar storage