If by "default" you mean that you selected Calendars->Default from the menu you just toggled the default calendar's visibility. You can enable the calendar by selecting it from the menu again.
If you really managed to reset the settings (you can do that from the "Reset all settings" button in the options dialog) then your settings are set to default values but your events are unaffected. If you had configured new calendars into Rainlendar you need to do that again in Options->Calendars.
Rainlendar also creates automatic backups every day so you can get all the previous settings back by restoring the backup file. The backup files can be found from Rainlendar's data folder
and the restoring can be done in the Manager dialog (File->Restore).