Has my iPhone been hacked?

All such messages are scams, 100% of the time, no exceptions.

Please read through: https://discussions.apple.com/docs/DOC-8071

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

Sorry, you will actually need to provide details.

Where did you see a warning that it was hacked?

In a web browser? If so, it is a scam.

In your calendar? If so, you subscribed to a spam calendar.

Now again, exactly what are you seeing. We have no idea what you are seeing on your device.

Unless you have previously jailbroken the iPhone, no, it is not hacked.

If you could provide any information, someone may be able to assist.

Again, what leads you to believe it is “hacked”?