Passcode has not been changed and phone is not accepting it

The passcode does not change itself. If you didn't change it, then someone else did. But, no matter what really happened, at this point you will need to reset the iPhone and then set it up again.

If your iPhone is running iOS 15.2 or higher, you can use the instructions linked just below which will not require the use of a Mac or PC:

How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

If you are not sure which operating system is installed on your iPhone, please use the link below, which will require the use of a Mac or PC.

If you forgot your iPhone passcode - Apple Support

If you have a backup of the phone, you can use the backup to restore the data on the phone during the new setup.

Something happened. We can argue about that later. In the meantime, we assume that you want to get up and running again.

You can contact Apple Support if you want to confirm our answers.