First of all, let's see one of the question from Yahoo!Answers:
How to recover lost contacts on iPhone 4?
I switched numbers (and data) between an iPhone 4 and iPhone 5
recently, and some of the contacts in the iPhone 5 got lost in the
process (I was trying to put them in the iPhone 4); is there any way to
retrieve those contacts??? I thought I backed up the stuff in iCloud.
Actually, there are two ways to get your contacts back, below I will introduce how to do it step by step for you.
Part 1: Learn how to recover lost contacts iPhone from iCloud backup
Though Contacts syncs through iCloud, Mac users can use the
inevitable syncing latency to their advantage and often retrieve deleted
contacts by going to the Contacts (or Address Book) application. This
also works with the iCloud web interface, and is best with recently
deleted contacts, or with devices that have been offline from iCloud:
# Disable the internet connection by pulling down the Wi-Fi menu and turn Wi-Fi OFF
# Launch Contacts (or Address Book) in OS X, or Contacts from
iCloud.com on the web, and use the search feature to locate the contact
# Open the contact and pull down the File menu, choosing “Export” then
“Export Vcard” to save the contact(s) as a .vcf file – this will serve
as a backup in case the next step does not work
# With the contact still selected, click the sharing button arrow and
choose “Email Card” to launch the default mail app with the contacts
# Turn Wi-Fi back on to send the email containing the contact card
# Go to the iPhone, open the email and choose the attachment, selecting to “Create New Contact”
The reason to turn off wi-fi quickly is to prevent Contacts from
syncing changes with the iPhone. If done fast enough, you will often
find the contact deleted from your iPhone is still sitting in iCloud.com
or Contacts app in OS X.
Part 2: Learn how to restore lost contacts on iPhone without backup
You need an iPhone data recovery program to if your iPhone lost contacts no backup, try the way below:
Step 1: Download and install program on your computer. Launch it and connect your iPhone with computer.
Step 2: Select “Recover Data from iOS Device” as recovery mode.
For iPhone 5S/5C/5/4S users, click the "Start Scan" button to scan your iPhone.
For iPhone 4 and iPhone 3GS users need to enter DFU mode to scan for lost files:
1) Hold your device and click the "Start" button.
2) Pressing the "Power" and "Home" buttons at the same time for exact 10 seconds.
3) Then release the "Power" button, but keep holding "Home" for another
15 seconds until you get the message of successfully entering.Whatever
the device you use, the program will automatically begin scanning your
iPhone for lost contacts after you doing the steps accordingly.
Step 3: When the scan is over, you can preview all found data before recovery. The last step for iPhone lost contacts restore is to choose "Contacts" to preview, mark it and click "Recover" to save contacts on your computer.