Check the following things and update if needed before making a new issue:
- PocketMine-MP version
- PHP binary version
- Plugin versions
When you have the latest version of those files, and the problem still occurs, making certain that a plugin is not the cause, then make a new issue on GitHub.
Give as much information as you can about when or what happened.
Did the server crash and generate a crash dump? Post the Crash ID in your issue report to help us diagnose the problem. If you do not have internet connection, you can paste the dump on Gist and post the link to it instead.