We had a case recently where iPhone contacts that were synced to an iCloud account suddenly disappeared. During the troubleshooting process the cause was unable to be determined. Either a glitch or via user interaction somehow the some of the contacts were lost/missing.
Logging into the web based side of the iCloud account confirmed the same - missing contacts on the web interface as well. There were no local backup of the contacts unfortunately.
Thankfully Apple provides a way to recover your missing contacts:
1. Log into icloud.com. Go to Settings:
2. Look for "Restore Contacts" towards the bottom under Advanced:
3. Now pick a contact 'set' that you would like to restore. Apple will replace all contacts and then it should sync down to your phone after the restore.
You should in most cases have a history of contacts going back around 1 month.
Hopefully this will help if you have misplaced your contacts.