mmmm, you might have moved the texture to a folder that its not supposed to be in.. In which case, (this will basically render any custom textures useless, for the moment), go rename your materials folder (materials2..) and then it will let the GCF's textures come through.. If the GCF textures dont work, then theres something else wrong.. (Note you have to restart hammer and compile to let the GCF textures to come through)