See Backing Up Your Forge Content Restoring a World Backup section.
If you are using Game Manager, then Forge would likely have exported a backup of the world for you automatically if you did not export one when prompted to upgrade your world for v11. The exported zip is likely in your downloads folder.
If you do not have a backup from v10, then we can likely figure out where the issue is coming from by checking the Foundry server logs again. It should tell us why the server cannot start.
If you do not have a backup, and the world data had already been migrated to v11 and can’t run on v10 anymore, then you will need to update your Foundry version to v11 and disable the modules which don’t work yet.
It’s very important to check module compatibility before upgrading your Foundry version if you rely on modules heavily 
It’s also very important to always make a backup of your world before upgrading.