The document's color management policy probably has been changed—see Color Settings.
The policy determines how new documents will handle the embedded profiles of placed images. For CMYK a setting of Off or Preserve Numbers (Ignore Linked profiles) will assign the document's CMYK profile to all placed images instead of the image's embedded profile—the CMYK profile affects the preview.
If you have Ask When Opening checked it's possible to change the policy of an existing document. So you could start with a policy of Off and later change it to Preserve and get different results placing the same image.
