What Apple told you is correct, but it would have been nice if they offered to continue helping you. I'll try to do that.
First question: To reset your Mac's password start by reading If you forgot your Mac login password - Apple Support, followed by If you can't reset your Mac login password. You will probably need to erase that Mac.
However, if that Mac has been associated with the iCloud Find My service, then you will be challenged for your iCloud / Apple ID account credentials. From what you wrote "someone" changed the phone number associated with that account. That's bad. Whoever that "someone" is put you in a world of hurt. Nobody but you should be able to make any change to your Apple ID account information: Keep your Apple ID secure - Apple Support.
So... your task now is to recover your own exclusive use of your own Apple ID.
First try signing in to your Apple ID account, here: https://appleid.apple.com/
If you can't do that, then you will need to initiate an account recovery request. Read How to use account recovery when you can’t reset your Apple ID password - Apple Support.
Some of these steps are addressed in If you think your Apple ID has been compromised - Apple Support, so review everything applicable.
