Some deleted database items are soft-deleted (unpublished) and can be restored, but only through the admin backend for now.
When you delete a note, task, or contact in SyneMind, it is not immediately and permanently removed. Instead, it is "unpublished" - it disappears from your normal views but the record still exists in the database.
A frontend self-service "Trash bin" restore feature is planned but not yet available in the regular user interface. Until then, you can go to the edit screen of a Note or Task, browse to your pages in the admin backend, and see a list of your items there - find the item (which will show as "unpublished"), open it for editing and click "publish" (not "Save draft").
Note: this does not apply to other database entries such as "quick notes" or "tasklets", or to uploaded files - once deleted, those are gone permanently.