What do I do if my MacBook has been hacked?

Probably a Scam

The The Built in Security  is all that is required to protect the computer.

Check in Safari Preferences >> Websites

Look in " Notifications " category on left.

Has the user Allowed a Certain Unknown Website to Offer Notifications ?

Look in " Pop up Windows " category on left.

Has the user Allowed a Certain Unknown Website to Offer Pop Ups ?

You ignore the BS that is trying to scare you into doing something stupid.

If you are browsing with Safari, Force Quit, then restart Safari while holding down the Shift key. Don't visit that web page you went to before.

The only way to hack your Mac is to be physically sitting at the Mac, or they trick you into installing something that will allow them remote access.

Effective defenses against malware and ot… - Apple Community

If you think your Apple ID has been compromised - Apple Support (CA)