● Google Calendar events
Events deleted in Google Calendar are kept in Google Calendar's "Trash" for 30 days. Open Google Calendar on your computer, click the Settings menu at the top right → "Trash", and click "Restore". For details, see Google Calendar - Delete an event (Restore deleted events in your trash).
Sometimes the events are not gone but simply not displayed. Please also check "Google Calendar events aren't showing or syncing".
● Events in the Local calendar
We're sorry, but Smart Planner has no feature for restoring deleted data. If you had created a "Google Drive backup", you can go back to the state at the time of that backup by restoring it (please note that any data you added after the backup will be lost).