I was setting up the command block system for my new Minecraft server, I started setting up the command which would destroy all dropped items, but I accidentally killed all entities! How can I undo this? Someone please help me.
You can't. Unless you have a recent backup to restore, this action is permanent. Next time, what I would do is test these sorts of systems in a backup of the world to begin with to try to spot any potential issues like this one, before implementing it on the live server.