There have been cases when hacked two-factor authentication?
The commonly used 2FA (30-second 6-digit codes) is basically just a random password that gets hashed with a time value to produce your code. So the advantage is that the password itself is never sent over the internet but it can still be leaked from the server or from your device via some malware or from backups.
That depends on type of 2FA you use. It's possible to use email as 2FA, even though it's rare nowadays.
There have been cases when hacked two-factor authentication?
There are many, but mostly happen due to social engineering. The victim is manipulated to give 2FA code which sent to their phone or generated by an application.