The world references mods that are no longer installed
Applies to Forge, NeoForge.
What it looks like in your log
Forge Mod Loader detected missing registry entries.
What causes it
Mods were removed or changed since this world was saved. Their blocks and items are missing and will be wiped if loading continues.
How to fix it
Reinstall the missing mods to keep that content, or back the world up first and accept the removal knowingly.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log