(I'm on third line ticket duty right now, so if you have something significantly more complex than RTFM, or an actual bug, you'll probably be talking to me this week)
The most common cause of getting locked out other than you abusing your fastmail account is having your credentials stolen and used for spam/fraud, which is why we recommend 2 factor authentication:
https://www.fastmail.com/help/account/securityupgrade.html
But yeah, even then we don't lock you out forever - just need you to reconfirm your identity and re-secure your account.