We're sorry for the trouble. What to do depends on which calendar is missing.
● Google Calendar events are missing
Make sure the same Google Account you used before is registered on the new device, then turn on "Calendar Settings" → "Link Google Calendar" in Smart Planner's settings screen. When the app asks for permission to access your calendar, choose "Allow", then check the calendars you want to see in "Calendar Settings" → "Visible Calendars". For details, see "Google Calendar events aren't showing or syncing".
● Events in the Local calendar are missing
Events in the "Local calendar" are stored on the device, so they don't move to a new device automatically. If you created a "Google Drive backup" on your previous device, open "Google Drive backup" on the new device, log in with the same Google Account using "Login with Google", and run "Restore From Backup". For details, see "How do I transfer my data to a new device?".