All the mods 6 v.1.8.3 Server crashed

Quote from Aternos

It looks like our system stopped your server because it took too long for it to start. This can happen when a server is overloaded (too many plugins, resource-heavy modpack, etc.). Our system automatically stops servers, if they don't respond within a certain amount of time, to prevent them from getting stuck during startup. Especially in the afternoon, when many servers run at the same time, servers running big modpacks tend not to start correctly.

Unfortunately, there is not much that can be done about this. You can only try to start your server again at another time or use a smaller modpack.