Contacts deleted

So did you log into www.icloud.com on a computer browser to see if the contacts were still there? If they are still there, then on the iPhone, go to Settings><your name>iCloud, and turn off Contacts. When it asks about keeping them on the iPhone, say no. Then power cycle the iPhone, go back to the same place and turn Contacts on again and wait for them to download. Do not do this until after you have logged into iCloud on a computer browser to ensure your contacts are still there.

How are you managing your contacts? Are you using iCloud or another email account? Contacts cannot just disappear if you have not done anything on the phone. First determine how you are managing contacts.