Managing personal Outlook.com account settings, security, and privacy
Hey there, CharlieIf your security information has changed, by someone else, you will need to submit a recovery form to Microsoft to prove ownership of the account and get back into it. I can see from the background, you are trying to sign into the Xbox app, which means you used the account on Xbox. For this reason, please can you complete the Xbox recovery account here: https://aka.ms/recovermyaccount
Please make sure you enter as much information as possible on the form. This will allow the Xbox account team to verify your ownership of the account.
Hope this helps,
Nathan