It's your turn
There is one very basic rule that you should always keep in mind: It's your task to keep your world saved all the time. Aternos tries to save your world, but the system has to manage thousands of servers every day, so there might be data losses. So if your world is lost for some reason and you don't have downloaded your world before, it's simply your fault.
Reasons why your world might get lost
Aternos has a dynamic system, which is mainly divided into two parts: The hosts saving the data, and the hosts executing your game servers. Thousands of servers are transferred every day between this two types of servers. The system behind this "cloud" of servers is completely developed by Aternos and may have malfunctions causing the loss of data in this system. Also, the hardware can have issues that are resulting in losing your server files.
Minecraft was in the beginning just a small game programmed by one person and is always growing since then. New features and optimizations may have bugs as they had many times in the past. These bugs may also damage your files like not saving your world data or anything else. Also, an overloaded server may cause problems in Minecraft.
Not only Aternos or Minecraft itself can fail on keeping your world saved, also you can make mistakes that delete your world. Actions like generating a new world, loading a backup of your world or reinstalling your server will delete your current world. These faults seem to be very dumb, but they happen more often than you expect.
Tips to keep your world safe
Backups: your task
Aternos allows you to connect a Google account to save backups in your Google Drive storage. You also can configure the number of automatic backups to be kept of your server. To learn more about how to create and manage backups, watch our YouTube tutorial. But there is another way to create backups of your world: The light blue "download" button. You can download your world on your computer at any time, even while your server is running. To keep your world 100% safe, we recommend downloading your world before stopping your server. And if you play on your server regularly, don't forget to download it from time to time. The more often you download your world, the smaller the loss will be.
Don't overload the server
Minecraft and our System don't like overloaded servers and there is a high possibility that your world gets lost when you overload your server. So don't play around with thousands of TNTs, don't place millions of blocks with WorldEdit, don't build gigantic complex machines in modpacks, don't use too many plugins and finally don't play on multiple worlds. Aternos is made for private use in a small environment and not for complex server systems in multiple worlds. The more worlds you use, the heavier the load on your server and the higher the risk of a crash.
As told before there are some ways, you can delete your world unintentionally with some actions. Aternos always warns before these actions so read all texts attentively. Also be patient when using the webinterface of Aternos. When you click a button like "stop" wait some seconds for a feedback e.g. a little message appears or the status changes. If you spam buttons you risk losses of your world.