They would start printing their own currency. Impossible to keep having the same currency at the same exchange rate if you leave. It's going to end up like Argentina when they wanted to keep a fixed rate with the USD. To be seen who's going to be Argentina and who the USA in that case.

By the way, this is the very problem with the Euro. If you want to have the same currency of (let's say) Germany, you should perform like Germany, or better. If you are less efficient, in any way, you're going the way of Argentina, even if not as quickly. This is why the EU should have one real central government, maybe USA style, and local governments should be like the governments of the US states.