What would happen if Banks involved in Money laundering were suddenly all hit with a virus that knocked out all their clients and accounts?

Probably a bit like when governments seize assets of people they label terrorists or criminals, except involving more people and probably only being somewhat temporary in most cases (because banks tend to have records and somewhat redundant systems).

What happens comes down to the specifics. Rarely do such entities have all their sources of wealth and power in one place. So while it has effects, it’s usually not enough to shut down such people completely, particularly when they have other access to power, and established patterns of behavior that they aren’t willing to give (and/or see no preferable alternative).

The question is peculiar because it mixes up attacks on banks with banks that are involved with money laundering.
Depending on how many banks are involved in money laundering (all of them? just convictions?) that could be a big chunk of the financial system. If those banks all go down for whatever the reason the economy will be in big trouble.