The best source to find problems with your server is the log file, which you can access here: https://aternos.org/log/
Reading a log is not easy, but learning it helps to identify problems with your server easier and faster. Log lines start with the time and the log level:
Time Log level
[13:50:55] [Server thread/INFO]: Starting minecraft server version 1.12.2
To identify problems with your server you will most likely look for errors, which you can find in lines with the ERROR log level:
[10:56:03] [Server thread/ERROR]: Encountered an unexpected exception
Sometimes the surrounding lines of an error provide further information regarding the error, so reading them might help. If you don't understand the error message you can try to google it to see if other servers had similar issues.
When there are any suggestions available, you will see them at the top of the page. You get automatic suggestions for errors in your log and buttons on how to solve those errors. Follow these suggestions to fix the problem.
If you still have problems starting your server, you can open a ticket here. Please make sure to include as many details as possible and also include the link to your log on mclo.gs. mclo.gs is a website developed by Aternos for simple log sharing, reading and analysing. Go to your log in the Aternos panel (https://aternos.org/log/) and click on the button "Share" in the top right. Then you get a field with the link to your log on mclo.gs.