Your server was running on Paper before, Paper as a Bukkit fork store the nether and end at a different location.
When you switch the software from and to Paper our system automatically corrects everything so you don't have the hustle.
In your case when you switched from Paper to Forge our system tried to do its job but found that there was already a nether world at the target path, for whatever reason... Maybe you uploaded a world?
It did nothing to make sure that you don't lose anything.
Long story short: Your nether should still exist but not at that location where Forge is looking for it.
To fix that:
- Turn your server off
- Create a backup, just in case.
- Go to the worlds page and click files on the main world.
- Delete the DIM-1 directory.
- Start the server.
Everything should be back then.