I just updated the server I've had for about 2 years to 1.15,
As I logged in, I noticed nothing special, I just played as I normally do. But then I went to the nether, I was confused because I didn't recognise where my portal came out. I thought my friend had put a new portal to troll me. Then it hit me, the ENTIRE nether had been reset. Our nether hub, network of portals and many farms all gone. Even after putting back 6 backups in both 1.14.4 (the previous version it was on) and 1.15, the nether did not come back. Very, very upsetting. 2 years of hard work is gone.
Was you using a server API before like Bukkit/Spigot/Paper and then using vanilla server SMP? API breaks worlds into separate folders, using Vanilla will regen DIM-1 and DIM1 folders. Do you have world_nether and world_the_end folders? If so, using a server API mentioned above will reconnect them as they divide worlds to separate folders.