Invisible geometry
 

for some reason every once in a while part of my geometry disappears. It still has the wire frame, but I can't see it. How do I fix this?
Thanks

check if back face culling is on.

also, make sure your normals are conformed so theyare all facing the same direction.

Also reaply a shader to it.

Can someone explain abit about the normals

in a nutshell, a surface normal is the direction the surface is facing.