Finance people sell insurance against things. It is an economic idea called “hedging”. If it would hurt your business if the Euro were to go down, you buy insurance contracts against that happening.
These are not insurance contracts you are used to, these are insurance contracts that are written generically, called derivatives. You can buy as many as you want, you can sell them back. The value of them changes minute to minute on the exchanges. People make fortunes buying these currency derivatives when, for example, early in the day people think the euro is safe and the insurance is cheap, then later in the day, people worry that the euro may fail and the costs of the currency derivative skyrockets.
Over time, these derivatives markets have evolved into huge betting pools. In fact, some financial institutions play with their books to use these derivatives contracts as collateral so they can borrow money to buy more insurance contracts to make bigger bets.
This is what happened to the US economy when the banks all failed in 2008, they had been betting on mortgages.
The currency derivatives securities market has a theoretical value of $3.27 trillion. If the Euro fails completely, that market falls apart overnight, causing most big European financial firms who have been trading in those things to fail, immediately, and some US firms who have been playing with them as well. 3.27 trillion is more than the yearly GDP of Germany.
Sorry, getting to be a long story. But their banks all fail. Any US banks holding their securities fail. The US Government has to step and bail out our banks again which are exposed, and help them bail out their banks.
Their business can’t afford to buy anything from America for awhile, and anything they have bought from the US they have been paying for on credit, they stop paying their bills, and the American company pretty much has to write it off as a loss.