If you use mods then please check if the versions of the mods match.
If you don't use mods and are facing the error for more than a few days please follow this article to help us and your ISP identify the problem: https://support.aternos.org/hc/en-us/articles/360027235671
Please read the article carefully and don't skip any steps, questions or suggestions.
Following this article will not instantly fix your problem. It only helps to find the root cause which can then be fixed by the network provider.
i dont use any mods its just an vanilla server