Server keeps crashing when I try to start it

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    I've been playing this server with some friends but yesterday at night we couldn't start up the server and keeps giving the same text in the log. https://mclo.gs/ce1vqA6

    Here it says something about a mod folder, however we are playing vanilla. Does anyone know how to fix it?

    Looks like you have removed few mods from the server which includes a mod called twilight forest. You removing a world generation mod is causing issue, so either add it back or regenerate the world

    My friends just told me that they deleted a mod... How can I get back the same world with everything we have done without corrupting the data?

    Nothing has happened to your world. Your friend's might have removed the world generation mod mentioned in my previous response. Just add it back or regenerate the world if you don't want to add it back. Regenerating the world will delete the data on the world