Aternos has many users around the globe. Unfortunately, we are not able to serve all users at the same time, because our resources are limited. Because of this, we have to stop empty servers without players.
You've probably noticed the small timer in the left corner of the green bar on the server page:
This timer tells you when your server will be stopped to free up the allocated resources for another user.
As soon as at least one player is on your server the timer vanishes and your server will stay online. After the last player has left your server the timer will come back and start to count down from the beginning.
The initial countdown time when your server is started is 5 minutes. After the last player has left the server, the countdown time is 3 minutes.