Bug i cant get mesh collider for my map working

so i very new to unity and ive spent a long time on a map and i think i have acidentaly deleted the collider
and i have tryed many tutorials like adding a mesh collider with a kinematic rigid body so doesnt move for more acurate collider and it doesnt work, i dont wanna go insane doing so many small details using box colliders so are there any more ways i can fix this?