If a bank is in danger of running out of money, then (in the US, where I work in banking) the federal regulators step in and take over the bank. They sell off assets to other banks and pay depositors their money (up to the EXTREMELY generous limits of FDIC insurance). The funds to pay for the losses come from an insurance system ("FDIC") that all banks pay into. (And in case of a widespread collapse of many banks at once, the federal government underwrites the FIDC insurance program.)