I'm sorry to hear you have lost the contacts on your iPhone.
If you are using iCloud, you can get them back easily.
Since your post doesn't mention iCloud, I'm including the information below to sync your contacts with iTunes.
Click on the link below to see more details and screenshots.
I've quoted some helpful highlights for you:
You can use iTunes to sync the contacts, calendars, notes, Safari bookmarks, and email account information on your computer with your iPod, iPhone, or iPad. iTunes also syncs this information from your device to your computer—for example, if you add contacts on your iPad, syncing adds those contacts to Contacts on your computer (or Address Book, in some versions of OS X.
You can sync information on your device with only one iTunes library at a time. When you sync, iTunes determines whether your device or your computer contains the most recent information, and lets you choose the information you want to sync.
Sync contacts and other info with iPod touch, iPhone, or iPad
- Connect your iPhone, iPod touch, or iPad to your computer and select your device.
- Click the Info button and select the items you want to sync.
- Click Apply.
I hope this information helps ....
Have a great day!