It could be a run on the bank, a computer network failure (either weather-induced or malicious cyberhacking), or simple insolvency. Banks fail all the time. It isn't dramatic, and you'll only know about it when you put your ATM card into the machine and it doesn't work.
If you want to know the future, look to Greece.
All I'm saying is, a bank is only one basket. Have other baskets, and many varieties of eggs. In other words, many currencies, not just the US dollar.
Stocks and bonds and cash (both in banks and in one's physical possession) denominated in Euros, yen, yuan, Swiss francs, and British pounds (or another currency that doesn't rise and fall in lockstep with the US dollar) wouldn't be a bad idea, and a little physical gold, silver, and platinum in a private safe (and perhaps a digital wallet full of Bitcoins) would make me more comfortable than a fat balance at Wachovia bank.
But that's just me.
Cows and pigs? Nah. Coffee, cigarettes, chocolate, whisky, gasoline and ammunition. Oh, and toilet paper. Plenty of toilet paper.