Why has my password been changed I didn’t change it

So if I understand you correctly you did not change your password but it is changed? Are you sure that you are typing it right?

You can use the links in the thread here but if you are sure you have not changed your password yourself, you should check the steps in this one:

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

Have you forgotten your password? -> See this Apple article: If you forgot your Apple ID password

https://support.apple.com/en-us/HT201487

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support