Think of it in terms of business lines of credit. Manufacturers (remember those?) get orders. They arrange in advance lines of credit with banks, through which they borrow money to buy raw materials to manufacture the product. After they sell the product they repay the line of credit loan.
If the bank thinks the manufacturer is in trouble and might not repay a loan they cut off the line of credit. The manufacturer won't have money to buy raw materials and won't be able to fulfill the order. If this happens most companies go out of business almost overnight.
Interbank lending is like lines of credit banks have with each other. Now does it make sense? If a bank doesn't have money on hand to fulfill its obligations it is out of business like yesterday. Look up counterparty risk
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