If your server shows Offline and won't boot, the answer is in Console, not a guess. Open it and read the last 10-20 lines before it stops. That names exactly what broke.
How to fix it
- Open Console and find the error at the bottom of the log. Note the exact mod or class name it mentions.
- If it names a missing dependency, install that mod through Mods Installer or Modpack Installer.
- If it says a mod "cannot be loaded on a dedicated server," that mod is client-side only, delete it from Files.
- If it mentions memory, see How Much RAM Does Your Modded Server Need?
- If it points at world data instead, restore a recent snapshot from Backups.
- Restart and check Console again. Fix errors one at a time, since dependency chains can surface one after another.
Add mods a few at a time and test between each one. Bulk-adding a modpack's worth of mods manually is the number one cause of crash logs that are hard to read.
Still stuck?
Removed a mod and now others are broken: many mods share a dependency, removing one can break several others at once. Reinstall it, or remove the mods that needed it.
Mod-specific errors: see Fix "Missing Dependency" and Mod Conflict Errors.
Log doesn't clearly name the problem, or you're on a legacy version with a startup script issue: open a support ticket with the last 30 lines of your log, staff can pinpoint it fast.
Did this solve it?
If you are still stuck, our team can look at your server directly.