A hacker has changed my passcode so now it doesn’t work anymore

You have a serious problem.

Perhaps these will help.

If you think your Apple ID has been compromised - Apple Support

See especially the first two bullets under “If your iPhone or iPad is lost permanently or was stolen“:

Find your lost iPhone or iPad with Find My - Apple Support

The only way a hacker can change your passcode is if they have physical possession of your iPhone and know the current passcode. It cannot be changed remotely or without physically holding the phone.

A hacker can change your Apple ID PASSWORD (not your phone’s passcode), but only if the password was a weak one that could be easily guessed, AND you do not have 2 factor authentication on your Apple ID account.