Lost contacts on my iPhone

Contacts are synced either with iCloud, or an email account on your phone. How to restore that has to do with how you are synced. What were you using to manage your contacts? Were you using iCloud? Were you using an email account such as Gmail, Exchange? Did you make any email account changes before your contacts disappeared? If you were using iCloud, go to Settings>your Apple ID>iCloud>Contacts, and makes sure that switch is turned on. Also see this, If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support