How to factory reset iPhone

Apple doesn’t post such messages to your phone. All such messages are scams from criminals.

Phony "tech support" / "ransomware" popup… - Apple Community

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

You have not been hacked. What makes you think that you were hacked? It might be a quick, easy fix that doesn't requiring wiping your device and losing all of your data.