Owner audit report?

Hello,

I am the administrator of a P1 plan with 25 users.

Due to poor password management of one of my users, an account was hacked, using the mailbox owner's username and password.  A lot of things were changed.

I need a log of the items that were changed.

Does this exist?  If this doesn't exist, is there a backup of this mailbox I can revert to?  If there is no backup, do you at office 365 offer a backup service I need to subscribe to?

Thank you,

Thomas