I had the same problem on a Windows server. My friends and I did not want to lose our world. I fixed it by downloading the latest Minecraft_server.exe. I renamed my original server folder from "Server" to "ServerOld" then created a New folder called "Server". Moved the new Minecraft_Server.exe to the new Server folder. From the ServerOld folder I copied the "world" folder into the new Server folder. Once I executed Minecraft_Server.exe all was back to normal again.
Hope this helps.
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Hope this helps.