Could a solar flare wipe out bank accounts?

No—an ordinary or even severe solar flare is very unlikely to permanently wipe out bank accounts or erase balances. The more realistic risk is temporary loss of access if a major geomagnetic storm disrupts power, telecommunications, banking networks, or the internet.

Banks and payment systems are generally designed with redundancy and backups, so customer records are not usually stored in just one vulnerable place. A solar storm could still cause outages, delayed transactions, or recovery problems, depending on how severe it is and how a particular institution is configured.

So the bottom line is: a solar flare could make it hard or impossible to use your money for a while, but it is not a realistic way to make your bank account disappear.