Can someone explain to me, how a hacker would attain an account on a website that is protected by 2FA with Google Authentication?

My havelock investments account was hacked.  I have 2FA authentication enabled, and yet, it was nothing to them.  How is this possible?

Also, my computer is virus free.  I don't download suspicious files and I definitely don't run them.  I would consider myself technologicaly savvy, especially enough not to be infected with something malicious.
Of course, after this happened, I scanned my whole computer with MBAM, ADwCleaner, and Avast.  They found nothing, nothing at all.