Apple Id and password has been changed and Not by Me

Are you currently logged into any Apple devices with your Apple ID? If so then you can simply go to settings in that device to reset the password. See this article: If you forgot your Apple ID password - Apple Support

If you aren't logged into any Apple devices you can find help with your Apple ID here: If you forgot your Apple ID email address or phone number - Apple Support

Once you figure out your Apple ID, the next step would be to regain entry to it... you will need to do account recovery at this point. Go to iforgot.apple.com to start the process. Please see this article: How to use account recovery when you can’t reset your Apple ID password - Apple Support

Apple Id and password has been changed and Not by Me

Who did change your Apple ID & Password?

If other people have access to your iPhone or other Apple device, change the device passcode and never share it with others.

Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

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

(if you haven't already done this) Two-factor authentication for Apple ID -> Two-factor authentication for Apple ID - Apple Support

To keep your Apple ID account secure, see the following Apple articles: 

If you think your Apple ID has been compromised

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

Recognize and avoid phishing messages, phony support calls, and other scams

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

See who has access to your device or accounts

See who has access to your iPhone or iPad – Apple Support (UK)

Delete suspicious content from your devices

Delete suspicious content from your devices – Apple Support (UK)

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

• if you see devices that don't belong to you, delete from your account.

Personal Safety User Guide -> Personal Safety User Guide – Apple Support (UK)

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)

Never jailbroken your iPhone.