Probably a crash while your server starts, but to troubleshoot it we need more information.
What is your Aternos username?
What software do you use?
Any error messages in the log? You can share the full log with us using the share button on https://aternos.org/log
What's that last status before your server stops? starting/loading or online?
Danosh, and I use Spigot, here is the log of my server :
[01:15:28] [Server thread/WARN]: java.io.FileNotFoundException: ./world/unknownplayers/Hinder863.dat (No such file or directory)
[01:15:28] [Server thread/WARN]: at java.io.FileInputStream.open0(Native Method)
[01:15:28] [Server thread/WARN]: at java.io.FileInputStream.open(FileInputStream.java:195)
[01:15:28] [Server thread/WARN]: at java.io.FileInputStream.<init>(FileInputStream.java:138)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.a(NameReferencingFileConverter.java:361)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.onProfileLookupSucceeded(NameReferencingFileConverter.java:338)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:78)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:409)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.DedicatedServer.aS(DedicatedServer.java:630)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:223)
[01:15:28] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:544)
[01:15:28] [Server thread/WARN]: at java.lang.Thread.run(Thread.java:748)
[01:15:28] [Server thread/ERROR]: Conversion failed, please try again later
net.minecraft.server.v1_12_R1.NameReferencingFileConverter$FileConversionException: Could not convert file for Hinder863
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.a(NameReferencingFileConverter.java:383) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.onProfileLookupSucceeded(NameReferencingFileConverter.java:338) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:78) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:409) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.DedicatedServer.aS(DedicatedServer.java:630) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:223) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:544) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[01:15:28] [Server thread/WARN]: Encountered a problem while converting the player save files, retrying in a few seconds
[01:15:33] [Server thread/WARN]: java.io.FileNotFoundException: ./world/unknownplayers/Hinder863.dat (No such file or directory)
[01:15:33] [Server thread/WARN]: at java.io.FileInputStream.open0(Native Method)
[01:15:33] [Server thread/WARN]: at java.io.FileInputStream.open(FileInputStream.java:195)
[01:15:33] [Server thread/WARN]: at java.io.FileInputStream.<init>(FileInputStream.java:138)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.a(NameReferencingFileConverter.java:361)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.onProfileLookupSucceeded(NameReferencingFileConverter.java:338)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:78)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:409)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.DedicatedServer.aS(DedicatedServer.java:630)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:223)
[01:15:33] [Server thread/WARN]: at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:544)
[01:15:33] [Server thread/WARN]: at java.lang.Thread.run(Thread.java:748)
[01:15:33] [Server thread/ERROR]: Conversion failed, please try again later
net.minecraft.server.v1_12_R1.NameReferencingFileConverter$FileConversionException: Could not convert file for Hinder863
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.a(NameReferencingFileConverter.java:383) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter$6.onProfileLookupSucceeded(NameReferencingFileConverter.java:338) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:78) ~inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.NameReferencingFileConverter.a(NameReferencingFileConverter.java:409) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.DedicatedServer.aS(DedicatedServer.java:630) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:223) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:544) inecraft_server.jar:git-Spigot-596221b-9a1fc1e]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[01:15:33] [Server thread/WARN]: **** DETECTED OLD PLAYER DIRECTORY IN THE WORLD SAVE
[01:15:33] [Server thread/WARN]: **** THIS USUALLY HAPPENS WHEN THE AUTOMATIC CONVERSION FAILED IN SOME WAY
[01:15:33] [Server thread/WARN]: ** please restart the server and if the problem persists, remove the directory './world/players'
[01:15:33] [Server thread/INFO]: Stopping server
[01:15:33] [Server thread/INFO]: Saving players