It's sort of like loaning money to a family member, we'll call Elmo. He goes out and spends it on his kids that just sit around playing video games. Basically, they're not all that good of a credit risk. They make some payment on the loan, but they keep on falling short. They start to max out all their credit cards, and they mortgage their house to the hilt. Elmo also has lost his job, and taken another that doesn't bring in as many bucks. So, the family keeps him afloat, but barely.
Eventually, someone says he has to pay up, and in order to do that, he has to reduce his spending. He starts checking the cushions for change, and even raids the kid's piggy banks. But, there just isn't enough to pay the creditors. But, Elmo keeps on spending. The money that you lent him was going to help pay for your kids education or a new car. But, that money is no longer available until Elmo pays up.
So, either you keep on lending Elmo money or he goes into bankruptcy. In order to stay afloat, Elmo decides to pay everyone in IOUs. In Greece's case, that would be another currency, if they drop out of the Euro. If relationships get really strained, Elmo may eventually be set adrift and not be invited to family functions.
So goes the sad story of Elmo. You're poorer, because your hard earned cash is gone, along with it's earning potential. But, then you realize that you've also lent money to your other family members like Betty, Portia, Lulubelle, and Issac. The worst of it is that you borrowed that money to lend to them. And they're talking about not paying, since Elmo didn't pay up.
I don't think a Greece default would bring down the Euro. It may devalue it, and that would make things expensive in Europe, and end up hurting the US.. But, the financial systems of the US and Europe are intertwined. It's similar to the banks back in 2008. Iceland and Cyprus have already declared bankruptcy, but they weren't in the EU. They had some very Draconian measures that they took, like confiscating what was in Cyprus banks and limiting withdrawals. That would pretty much crush the Euro if it started a contagion that spread to the rest of the PIIGS. Whatever they do, it will be on a weekend when the banks are closed. Cyprus did it, so did Lehman Bros.