I found one of my reminders disappeared and I doubt if I deleted it accidentally. Can I restore it if I accidentally deleted it?
If it was present when you last backed up the phone, restoring the backup will restore the reminder (along with everything else as it was at the time of the backup).
iOS: How to back up and restore your content
Did you happen to look in the completed list? If you accidently tapped the reminder in the list, it would move to completed.
Retrieving data ...