Customers of Silicon Valley Bank (SVB), a commercial bank headquartered in Santa Clara, California.was the 16th-largest bank in the United States at the time of its failure,  withdrew $42 billion from their accounts on within one day. which is the largest and fastest in history

The classic definition of a bank run is when an excessive number of clients simultaneously withdraw the whole balance of their deposits from a banking institution out of concern that it may be or will be bankrupt. Although many jurisdictions have deposit insurance programs to protect depositors, a customer’s trust in the organizations they deal with may sometimes be outweighed by their incapacity to use their money or a potential loss (beyond insurance coverage).

Bank runs happen when customers panic and everyone tries to get their money out at once. 

No bank in the world could survive that kind of outflow in a single day, especially considering that a comparable exodus of that scale the following day was all but guaranteed. SVB is now a state-mandated ward as a result, and it is frantically trying to sell before Monday.