Glitched or Hacked?

[Illetti]

Alot of hard work went into our server (Lipssss) and we logged on today to find alot of the buildings with large areas missing either the server's been hacked (We have a whitelist) or its Glitched out, is there a way to roll it back to a previous point? 

EDIT

I also noticed a user by the name of Zgell on the server user cache that wasn't a whitelisted member, so I believe we were backdoored or hacked into 

I noticed this in the files 

[07:57:16] [Server thread/WARN]: **** SERVER IS RUNNING IN OFFLINE/INSECURE MODE!
[07:57:16] [Server thread/WARN]: The server will make no attempt to authenticate usernames. Beware.
[07:57:16] [Server thread/WARN]: Whilst this makes it possible to use BungeeCord, unless access to your server is properly restricted, it also opens up the ability for hackers to connect with any username they choose.
[07:57:16] [Server thread/WARN]: Please see http://www.spigotmc.org/wiki/firewall-guide/ for further information.
[07:57:16] [Server thread/WARN]: To change this, set "online-mode" to "true" in the server.properties file.
[07:57:16] [Server thread/INFO]: Preparing level "world"

Edited August 18, 2020 by Illetti

[_Tarna_]

It will say the offline mode thing on every server. It is needed for BungeeCord. The player could have maybe joined before you whitelisted it or something. Also the only way you can rollback to a previous point is if you have a backup of the world downloaded, or a plugin called CoreProtect. If you get it now, it won't help but it will help if anything happens in the future.