Issue Description
Some or all contacts disappear from the Contacts app.
Common Causes
iCloud sync disabled
Wrong account selected
Account sync error
Step-by-Step Solution
Step 1: Check Contacts Account
Settings → Contacts → Accounts
Ensure iCloud or correct email account is enabled
Step 2: Enable Contacts Sync
Apple ID → iCloud
Turn ON Contacts
Step 3: Restart iPhone
Refreshes contact sync services
Step 4: Re-add Account
Remove and re-add email or iCloud account
Explore Further
Check contacts on iCloud.com
Prevention & Best Practices
Use one primary contacts account.