If you accidentally deleted iCloud reminders
If you accidentally deleted iCloud reminders, you can use iCloud.com to restore them, together with your calendars and events, from an earlier version that was automatically archived in iCloud. When you restore an earlier version, your current version is first archived in iCloud so you can change your mind and go back to it.
Note: If you have shared reminder lists or calendars, all sharing information is removed when you restore calendars and reminders. You have to share your reminder lists and calendars again, and ask other people to reinvite you to share their reminder lists and calendars.
For instructions, see Restore your contacts, calendars and reminders, or bookmarks in iCloud Help.