SVG imported all one colour or flat

First check whether the file contains separate vectors or a single raster image.

Check what is in the file

Run the checker and look at shape, group and raster findings.

A black preview alone does not prove the layers disappeared; inspect the destination’s layer list.

Keep an editable original.
For a raster source, trace it if an approximate silhouette will serve the job.
For separate vectors, ungroup the import and assign colours or operations in the destination.