Apple ID hacked

Hello,

My apple id got hacked by someone, email received by apple said that changes as been made but it doesnt show what changes were made, then suddenly it shows a notif that someone wants to add an email to my imessage, i declined and go try to change my password which was unsuccessfully since it says that its invalid. Now ive contacted apple they were useless tbh to solve my problem and get it back. So i have important stuff on it and many university document. How to access those important information and documents? and are there any way to get back my apple id since i know the email that was trying to add itself on my imessage i mentioned above. Moreover, i tried to open one of my documents for uni on my desktop and it doesnt open (it show the download sign) which means its on my icloud.