Banks have reserve money for situations like these. Although of course a user wouldn't be able to get the money out of their savings accounts from the get-go as paperwork and processing days would need to be taken into consideration. But it's not gonna be used to track the people to which they lent your money onto.

Banks could lose money when they start to lose money in their investment ventures. As you know banks finance houses, cars, and basically every commodity that could generate income long as it's not perishable. When a debtor, someone who owes them money fails to pay for let's say, the house that they gave him to finance for, they have the ability to repossess said property and either put it up for sale or rent it out to other people. This is called a foreclosure of a property. Now, in very rare occasions when a bank couldn't find suitable, well-paying buyers nor capable customers who are interested in their offerings, their investments will slowly depreciate in value until they are worthless. This is but one of the most common (yet still rare) ways that a bank could lose their "money". But the thing is that for instances like these the government usually buys out whatever property they are at a loss on and gives them money in return, sometimes the government doesn't even take the property and just gives them money all because banks are an essential part of the capitalistic system.