How I recover contacts lost after 15.6 iOS update?

If you sync contacts with iCloud you may be able to recover them in iCloud.com using the instructions in here.

Restore contacts stored in iCloud on iCloud.com - Apple Support

If you don’t sync contacts with iCloud, but you include them in your backups, you can restore a backup to recover the contacts. Restoring a backup will overwrite all of the current contents on you device and replace that content with the content in the backup.

See this for help with restoring a backup.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you sync contacts with an email account, make sure that you have that enabled in the email account. Settings>Mail>Account>Account Name>Contacts>On.

They are in the same place they were before.

Settings, Contacts, Accounts

ensure the correct account has Contacts toggled on.