A small country uses a digital economy, meaning there is no cash used in daily transactions. All transactions are stored on a government's central database.
An EMP attack/blackout manages to take down the nations power grid and internet. Now the nation has no way to trade for goods and services.
How can citizens continue to trade during this this temporary problem? Whether a short-term blackout or a prolonged one lasting days?
If paper and pens still exist in this world, then physical invoices can be used as transaction records during the blackout.
An invoice is a document that records all info related to a transaction, such as the agreed price, the items sold, and when the payment needs to be made. Today, both electronic and physical invoices are used for transaction where payments are not immediate(source: I develop transaction software), such as large-scale purchases of bulk goods. They are typically issued by the seller to the buyer, so many businesses would already be familiar with generating invoices.
So, all that is needed to temporarily replace the electronic economy is a steady supply of paper and pens. The seller would write the goods being sold, the amounts, the prices, etc., and the buyer and seller would both sign/somehow mark the invoice in order to authenticate it. Two copies, one for the buyer and one for the seller, would likely be made. When the electronic systems come back online, these invoices would be used to ensure that the proper money transfers are made.
Backups
Having backups in protected sites will make sure the economy goes back to normal as fast as possible, just make sure you have a concise and detailed plan to restore the backups.
Bartering
Well, things got really bad and the backups will take too long to get back online or they don't exist. Well, its time to bring back the chickens and start bartering.
Gift-Economy
If we are talking about a small nation, then their best bet would be to transition to a gift-economy or some similar model. Without anymore money on the country, then things and acts will have worth.
In the short term you will have people bartering for food, shelter, and manufactured goods. In the long term you will want to start a decentralized production grid where each village could sustain itself.
To expand on the answer by Albert:
Once upon a time, there was a mostly cash-based economy for the equivalent of $100 or less, and some sort of check for larger amounts. Then the "plastic money" became more and more prevalent -- first in the supermarket for the groceries, then at the bakery, finally at the hot dog stall for a quick snack.
But at first cash would simply become uncommon without being phased out. Small kids still get their pocket money in cash. One might give a coin to a beggar in the street. Things like that.
At some point, the government advises the citizens that a cashless economy is fine, but they should keep a little bit of cash in a safe place. Just as many governments advise their citizen to keep food and water for a few days at home, and candles and matches, and a battery-powered radio.
So sober, respectable citizens have an envelope with some mixed-denomination bills somewhere in their desks, and most still have a handful of coins somewhere.
When the blackout comes, those reserves are not nearly enough, any interesting stories arise.
Some of my ideas:
