MAJOR bug : around a third of the workspace suddenly stops working

Everything worked well, then when I open the workspace the next day, BOOM everything went away suddenly, so I hope my post helps you fixing this bug.

Maybe it is a set of many bugs, because the majority of the error messages are : 

"error: package AncientlegendsModItems does not exist"

 when the item does exist in the workspace.
These errors are only related to items.

then there's these ones (these elements used to compile and work perfectly fine) which are related to procedures (that were recently added or modified) :

Also there's an issue with the mod ID for some reason :

error: cannot find symbol public static final DeferredRegister<CreativeModeTab> REGISTRY = DeferredRegister.create(Registries.CREATIVE_MODE_TAB, AncientlegendsMod.MODID);