What do you mean with "closing"?

And what kind of server did you use? Is it vanilla, did you set up a snapshot server or did you use something such as Spigot or Forge?

Anyway, first place to look is the logfile. Check the 'logs' folder for that, it probably contains a file "latest.log', that file should tell you more about what it happening.

Also take note of any error messages, and share them. Because without those all we can do is guess.