Whenever you complete a worker job, you will lose 4 percentage points of energy. If you have under 4% energy, you are unable to work. You can recover energy by consuming Food Packages. To find a retail store, go to the World Map and select a country, a city and a piece of land with a retail store built on it. A piece of land with a retail store built on it is denoted with a shopping cart icon, as shown below.
On the land page you can see the store's current offers, if there are any, and you can select an amount of items to buy. Note that if you buy items from a retail store that is in a different city or country than your current home, or if you are homeless, you will have to pay a fixed delivery fee.
Once you have bought items from a retail store, they will appear on your Dashboard » Money & Items page. Click on "Use" to consume a Food Package or Medicine Box. Each Food Package recovers 20% energy, and each Medicine Box recovers 20% health.