Troubleshoot an STL file that will not open
Isolate the file from the display
Use Sample in the lower right of the viewer. If you can see and rotate the impeller, the browser has rendered a known mesh. Try opening your own file again and read the message below the viewer. A failed import can leave the previous model visible, so check the filename before assuming you are looking at the new part.
Check the file itself
Extract compressed downloads before opening them. A binary STL has a triangle count followed by triangle records. An interrupted transfer may leave some records missing. An ASCII STL describes facets using text, and each facet needs three valid vertex coordinates. If an export appears incomplete, get another copy from the source application. Renaming a CAD drawing or a ZIP archive with an STL extension does not convert it.
Reduce a mesh that exceeds the limits
Check the file size against the 64 MiB limit and the triangle count against one million. For a very large text STL, a binary export of the same mesh may solve the file size problem. To reduce the triangle count, adjust tessellation at export and inspect important curves again. If the viewer reports the 100,000-triangle fallback limit, use a WebGL-capable browser or export a simpler mesh.