After the Team understood the Blockchain System, where everything is public, and no one can store secret information, They made another version.
The new version BankV2 is simpler. They made a simple Bank system that allows accepting and withdrawing ETH, the project is life and users starts using it.
The Bank Contract Ether increases, as users are depositing, but Bank Developers noticed that the Bank Ether balance is zero. They investigated the issue and found that just a single user withdrew his balance, but other users still had a balance in the Bank.
You need to know what is the problem that makes the Attacker being able to drain the Bank Balance.