Was S-Planner set to sync with an online service? Most usually, I'd expect this to be your Google calendar. If so, check the online service in a browser - if the data's there, it's just an app issue and it may be possibel to re-sync it (there's probably an option either in the app settings or the system sertings under Accounts & Sync. You could also try deleting the data in Settings|Apps, though I'd back up the online data first (in Google, that should be on your dashboard).
If the data was only on the phone, you may have lost it altogether.
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