What is it about banks?

www.bbc.co.uk/news/business-18284747

The link is to a typical article in the mainstream media. It purports to explain the financial crisis and it has a nice picture. But unfortunately it does not explain anything at all, to me.

The danger appears to be runs on the banks: that is when the people who keep their money there ask for it back, all at the same time. The banks don't have the money, so they can't pay and the bank collapses.

Either the money exists or it doesn't. Apparently it doesn't. My problem with this is that I cannot see what has changed: the money never did exist. If we stop believing in fairies they die. But there is no difference in the real world if all the fairies die, and I cannot see why there should be if all the banks die. There would be some horrible confusion while people adjusted, to be sure. But folk would still be making stuff and moving it about.