If people in general come to believe that banks have insufficient money then they may all decide to

A bank run which is known as a run on the bank occurs in a fractional reserve banking system when a large number of customers withdraw their deposits from a financial institution at the same time and either demand cash or transfer those funds into government bonds, precious metals or stones, or a safer institution because they believe that the financial institution is, or might become, insolvent. As a bank run progresses, it generates its own momentum, in a kind of self-fulfilling prophecy (or positive feedback loop) – as more people withdraw their deposits, the likelihood of default increases, thus triggering further withdrawals. This can destabilize the bank to the point where it runs out of cash and thus faces sudden bankruptcy. Several techniques could be used to try to prevent or mitigate the effects of bank runs. The government authorites might order the bank to suspend its operation temporarily, so that the series of withdrawls could be stopped to prevent a bank run.