On Aternos, each server is limited to 4 GB of storage space for technical reasons. If you reach this limit, you are no longer able to start your server.
There are a few things you could try to reduce the size of your server:
Delete unused Chunks
Minecraft generates thousands of chunks around players, and saves them, even if no player has ever entered them. To automatically remove unused chunks, enable the Optimize option for your worlds.
Delete unused worlds
Worlds usually make up the most of a server's total size. If your server has multiple worlds, and you do not need all of them, you can delete the unused ones here: https://aternos.org/worlds/
If you are running a Vanilla, Forge or Fabric server, you can delete/reset unused dimensions by deleting their corresponding folders within your world files (e.g. "DIM-1" for Nether, "DIM1" for End).
Delete unused mods/plugins
Mods and plugins can also take up a lot of space. Therefore, you could try to reduce the size of your server by deleting some of them from your server files: https://aternos.org/files/
When deleting plugins, make sure to remove both the plugin's .jar file and folder from the plugins directory.
Delete unused region files
You can also remove unused parts of your world by removing their region files from your world's "region" folder.
To find out which file corresponds to which part of your world, you can use this online tool: https://dinnerbone.com/minecraft/tools/coordinates/
If you can't further reduce the size of your server yourself, our support team might be able to help you.