If Aternos keeps crashing and the server will not start, then the issue is most likely a broken mod, or one of your mods needs a support mod. So, if you want to fix the modded server not starting, please check out your logs if there is a broken mod. If you find one, then just delete that specific mod.
However, for those who are using a mudpack, there is not much that you are able to do since you cannot delete a separate mod in the mudpack. Starting your server on the site called Aternos is normally just as easy as clicking the start button and allowing the system of the site to do the hard work of loading all the files and starting the server with the right configuration. Sometimes, that is not enough and the server refuses to start and just switches the status from starting to stopping after the Aternos system discovers the failed start.
When this issue happens, it is most likely of the times of fixable issues caused by the installed software and not an issue in the Aternos system. The best one to find the issue is the log file of the server, which you are able to access on https://aternos.org/log/.
For most people, reading a log is not easy, but learning it helps to identify the issues with your server easier and also faster. Apparently, log lines begin with the time and the log level:
|[13:50:55]||[Server thread/INFO]: Starting Minecraft server version 1.12.2|
In order to identify the issues with your server, you have to look for the errors, which you are able to find in the lines with the error log level:
|[10:56:03]||[Server thread/INFO]: Encountered an unexpected exception|
In some cases, the surrounding lines of the error provide further information about the error, so reading them might help. For those who do not understand the error message you are able to try to look for it on the internet to see if the other servers had similar issues.
When there are some suggestions available, you will be able to see them located at the top of the page. You can get the automatic suggestions for issues in your log and buttons on how to fix those errors. do not forget to follow these steps to fix the problem.
In case you still encounter issues starting your server, you are recommended to open a ticket. Please be sure to include as many details as possible and do not forget to include the link to your log on mclo.gs. For those who are not familiar with mclo.gs, this one is the name of a website developed by Aternos for simple log sharing, analysing, and reading. You can go to your log in the Aternos panel (https://aternos.org/log/) and press on the Share button that is able to be found in the top right. Then, you will get a field with the link to your log on mclo.gs.