When high poly 3D models are invisible, start by checking three common issues: flipped normals, corrupted mesh data, or enabled backface culling in your software.
Flipped normals (faces pointing the wrong direction) are a top cause—use your software’s "recalculate normals" tool to fix. Corrupted meshes can be repaired with "clean up" functions, and backface culling (which hides reverse-facing faces) should be toggled off to test visibility.
If problems continue, isolate the issue by hiding parts of the model or check if the material’s transparency is accidentally set too high (lower it to restore visibility).