My server which is unlisted and private was griefed by someone whose username is "ghsx". They held this rocket looking thing and it left behind a trail of what would look like TNT explosions except instant. Since my computer is very slow and laggy they went on for about 5 minutes as I tried to ban them. I eventually did but not after destroying hours of work from multiple players. The original game mode someone is set to is creative because it's more of building server but I now set it to survival and opped those who I trusted and who had been building on the sever. I don't know if there is anyway to get back what was lost or to prevent something like this from happening again.
