Failed to load level.dat

[ERROR]: Failed to load level.dat
java.io.IOException: Failed to read NBT

level.dat stores the world settings: seed, spawn point, gamerules, time. Without it, the world won't load.

Recovery

Next to it sits level.dat_old — a backup copy of the previous state:

1. Stop the server.
2. Rename level.dat to level.dat.broken.
3. Rename level.dat_old to level.dat.
4. Start the server.

This recovers the world in 90% of cases. You'll lose only the most recent world setting changes, not your builds: chunks are stored separately.

If there is no backup

• Restore a backup — the fastest route.
• Or create a new world with the same seed and move the region, entities, data folders into it — World transfer. Your builds will be preserved, but you'll have to set the settings again.

Causes of corruption

A hard shutdown while writing, a full disk, copying files on a running server — Corrupted world, Disk full.