Hello!
Unfortunately, that's something that occurs on your host's side.
There's a million things that can cause this. I'll try not to get too technical, but it happens when the java virtual machine (aka, the thing that's essentially running your spigot/bukkit server) gets disconnected.
There's a lot of things that can cause this, but it's most commonly caused when the panel you're on throttles connections or runs out of memory. In other words, the panel has too many servers running on it, it get's overwhelmed and stops responding to some of them.
If you have a host like GGservers or MCProHosting, you're a lot more likely to run into this, as they put WAAAAY too many servers on each panel. Remember, each panel is a computer with a limited amount of ram and power (make no mistake, they have a lot of RAM and a ton of power, but it still has its limits). When (for example) you try to run 150 servers on a panel that only has 64gb of RAM, you might see something like this happen.
Again, there's really not a whole lot you can do to fix it, as it's your host.
