Several users have reported that all or some of their contacts disappeared after iOS update. If you consistently encounter this issue, try the following steps to resolve your issue. Here is how to recover lost contacts on an iPhone:
Note: Before trying these tips, please upgrade your iPhone to the latest iOS firmware.
iCloud stores your contact information. You may sync your contact info only if you had enabled iCloud Sync with Contacts enabled before losing your contacts (Settings > iCloud > Contacts). (How To Set Up iCloud)
Tap Settings > iCloud > Contacts and turn off your Contacts and turn back on. This will trigger a re-sync with iCloud to get your contacts back.
Import from SIM
If you are storing contacts in your SIM card, then you may try this option:
Settings > Mail, Contacts, Calendars > Import SIM Contacts
Connect your iPhone to iTunes and sync your contacts.