How can I recover an account if it no “longer exists”?

My sister is having problems with her Microsoft account. Her account "does not exist". When she tried, signing into her account she ws notifyed to by microsoft that it seemd that someone was trying to get into her account. She had to fill out some security questions to enter. When she entered her account, she was redirected to a microsoft accounts page to change her main alias. When the she changed it she realized that her account is registered in important pages such as the SAT, TOEFL, etc, and therefore these could not contact her through her your old account,. She received an email to another account to confirm the change of alias, she clicked to cancel, and the account closed. It no longer exists, neither the alias nor the main account. She spent many years with this account and contains extremely necessary information that she needs to recover, not so much contacts as such, but information about accounts linked with this email.

I appreciate the help in advance,