Hi @INUMAKI TOGE ,
Suddenly???
There can actually be a number of things that can cause problems with the Boolean Modifier.
My wild guess would be: inconsistent Normals.
How to fix that depends on the Blender Version you are using; in Edit Mode, select everything and CTRL+N (‘old way’) or SHIFT+N (‘modern way’).
If the Boolean Modifier suddenly stops working (it’s working fine, you go to the toilet and when you come back it doesn’t work anymore), you should update your OS, all your drivers and maybe even buy a new computer.
.
