How to retrieve deleted contacts

That would depend on how you are managing your contacts. Contacts are synced with a mail account, or with iCloud. Once you determine that, you will know where to look. Did you lose all contacts? If that is the case, did you remove an email account from your device? Check Settings>Contacts>Accounts and see which accounts listed there have the word Contacts listed under them. If it is an email account, go to the web access of that email account and see if your contacts are still listed there. If so, go back on the iPhone and see if you have Contacts turned on for that account. If iCloud, check Settings>your Apple ID>iCloud and see if Contacts is turned on. If it is iCloud, see this for recovering iCloud contacts. Restore contacts stored in iCloud on iCloud.com - Apple Support