Once you delete any thing from iPhone, it is not possible to restore.It is possible to restore only when you backuped the contacts before deleting them.
So,if you backup the contacts before deleting then follow the below instructions to recover lost contacts from iPhone.
---> Turn on the iPhone device.
---> After turn on locate the backup application which you are using and open it.
---> After open you will see the different options.Among this select and tap the restore option.
---> Now select the contacts option.
---> Here you will see the all contacts.
---> Among this select the all contacts and tap the restore option.
---> After restore process completes check the contacts restore or not and close the backup application.