I got a message saying “your iPhone has been hacked”

Loads of people get this message and it's 100% a SCAM. Ignore it. Your phone has NOT been hacked. It's almost impossible to hack an iPhone and these messages are designed to scare people who don't know an iPhone can't be hacked to pay them for tools services, they don't need, for a problem they can't have.

Your phone has not been hacked.

The message is a scam that is trying to trick you into providing your credit card details to buy useless software, or worse.....getting you to reveal personal details about yourself like passwords, etc.

Ignore and delete message. Do the same for other messages like this that you will see. There will be more for sure.